feat: custom inline highlight based on prefix

## Details

Request: #544

Allows setting custom inline highlight groups to use for the associated
text body based on matching a configured prefix. When a matching prefix
is found it is also hidden since it adds only metadata. For example the
following will apply a red background to inline highlight text that
starts with an exclamation mark:

```lua
vim.api.nvim_set_hl(0, 'Red', { bg = '#ff0000' })
require('render-markdown').setup({
    inline_highlight = {
        custom = {
            important = { prefix = '!', highlight = 'Red' },
        },
    },
})
```

No defaults are setup for this out of the box for the time being, it
simply allows users to set their own values, this may change.

Author: MeanderingProgrammer

README.md

@@ -842,6 +842,11 @@ require('render-markdown').setup({
         render_modes = false,
         -- Applies to background of surrounded text.
         highlight = 'RenderMarkdownInlineHighlight',
+        -- Define custom highlights based on text prefix.
+        -- The key is for healthcheck and to allow users to change its values, value type below.
+        -- | prefix    | matched against text body, @see :h vim.startswith() |
+        -- | highlight | highlight for text body                             |
+        custom = {},
     },
     indent = {
         -- Mimic org-indent-mode behavior by indenting everything under a heading based on the

doc/render-markdown.txt

@@ -908,6 +908,11 @@ Default Configuration ~
             render_modes = false,
             -- Applies to background of surrounded text.
             highlight = 'RenderMarkdownInlineHighlight',
+            -- Define custom highlights based on text prefix.
+            -- The key is for healthcheck and to allow users to change its values, value type below.
+            -- | prefix    | matched against text body, @see :h vim.startswith() |
+            -- | highlight | highlight for text body                             |
+            custom = {},
         },
         indent = {
             -- Mimic org-indent-mode behavior by indenting everything under a heading based on the

lua/render-markdown/health.lua

@@ -6,7 +6,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.9.11'
+M.version = '8.9.12'
 
 function M.check()
     M.start('versions')

lua/render-markdown/render/inline/highlight.lua

@@ -20,14 +20,23 @@ function Render:run()
     for _, range in ipairs(self.node:find('==[^=]+==')) do
         -- hide first 2 equal signs
         self:hide(range[1], range[2], range[2] + 2)
+        -- hide last 2 equal signs
+        self:hide(range[3], range[4] - 2, range[4])
+
+        local custom = self:custom(range, { 0, 2, 0, -2 })
+        local highlight = self.config.highlight
+        if custom then
+            -- hide custom prefix
+            self:hide(range[1], range[2] + 2, range[2] + 2 + #custom.prefix)
+            highlight = custom.highlight
+        end
+
         -- highlight contents
         self.marks:add(self.config, false, range[1], range[2], {
             end_row = range[3],
             end_col = range[4],
-            hl_group = self.config.highlight,
+            hl_group = highlight,
         })
-        -- hide last 2 equal signs
-        self:hide(range[3], range[4] - 2, range[4])
     end
 end
 
@@ -42,4 +51,26 @@ function Render:hide(row, start_col, end_col)
     })
 end
 
+---@private
+---@param range Range4
+---@param offset Range4
+---@return render.md.inline.highlight.custom.Config?
+function Render:custom(range, offset)
+    local lines = vim.api.nvim_buf_get_text(
+        self.context.buf,
+        range[1] + offset[1],
+        range[2] + offset[2],
+        range[3] + offset[3],
+        range[4] + offset[4],
+        {}
+    )
+    local text = table.concat(lines, '\n')
+    for _, custom in pairs(self.config.custom) do
+        if vim.startswith(text, custom.prefix) then
+            return custom
+        end
+    end
+    return nil
+end
+
 return Render

lua/render-markdown/settings.lua

@@ -1094,6 +1094,11 @@ M.inline_highlight = {}
 
 ---@class (exact) render.md.inline.highlight.Config: render.md.base.Config
 ---@field highlight string
+---@field custom table<string, render.md.inline.highlight.custom.Config>
+
+---@class (exact) render.md.inline.highlight.custom.Config
+---@field prefix string
+---@field highlight string
 
 ---@type render.md.inline.highlight.Config
 M.inline_highlight.default = {
@@ -1106,12 +1111,25 @@ M.inline_highlight.default = {
     render_modes = false,
     -- Applies to background of surrounded text.
     highlight = 'RenderMarkdownInlineHighlight',
+    -- Define custom highlights based on text prefix.
+    -- The key is for healthcheck and to allow users to change its values, value type below.
+    -- | prefix    | matched against text body, @see :h vim.startswith() |
+    -- | highlight | highlight for text body                             |
+    custom = {},
 }
 
 ---@return render.md.Schema
 function M.inline_highlight.schema()
+    ---@type render.md.Schema
+    local custom = {
+        record = {
+            prefix = { type = 'string' },
+            highlight = { type = 'string' },
+        },
+    }
     return M.base.schema({
         highlight = { type = 'string' },
+        custom = { map = { { type = 'string' }, custom } },
     })
 end
 

lua/render-markdown/types.lua

@@ -194,6 +194,11 @@
 
 ---@class (exact) render.md.inline.highlight.UserConfig: render.md.base.UserConfig
 ---@field highlight? string
+---@field custom? table<string, render.md.inline.highlight.custom.UserConfig>
+
+---@class (exact) render.md.inline.highlight.custom.UserConfig
+---@field prefix? string
+---@field highlight? string
 
 ---@class (exact) render.md.latex.UserConfig: render.md.base.UserConfig
 ---@field converter? string|string[]