feat: min_width code block property

## Details

Adds `code -> min_width` which can be used to set a minimim width for
block width code blocks. This functions in addition to right padding.
Makes it easy to create uniform width code block sections.

Other changes are a change to the handler add methods to flatten the type
structure of marks and shorten things a bit.

Author: MeanderingProgrammer

README.md

@@ -263,6 +263,8 @@ require('render-markdown').setup({
         left_pad = 0,
         -- Amount of padding to add to the right of code blocks when width is 'block'
         right_pad = 0,
+        -- Minimum width to use for code blocks when width is 'block'
+        min_width = 0,
         -- Width of the code block background:
         --  block: width of the code block
         --  full: full width of the window
@@ -547,6 +549,8 @@ require('render-markdown').setup({
         left_pad = 0,
         -- Amount of padding to add to the right of code blocks when width is 'block'
         right_pad = 0,
+        -- Minimum width to use for code blocks when width is 'block'
+        min_width = 0,
         -- Width of the code block background:
         --  block: width of the code block
         --  full: full width of the window

doc/render-markdown.txt

@@ -295,6 +295,8 @@ Full Default Configuration ~
             left_pad = 0,
             -- Amount of padding to add to the right of code blocks when width is 'block'
             right_pad = 0,
+            -- Minimum width to use for code blocks when width is 'block'
+            min_width = 0,
             -- Width of the code block background:
             --  block: width of the code block
             --  full: full width of the window
@@ -579,6 +581,8 @@ CODE BLOCKS                                *render-markdown-setup-code-blocks*
             left_pad = 0,
             -- Amount of padding to add to the right of code blocks when width is 'block'
             right_pad = 0,
+            -- Minimum width to use for code blocks when width is 'block'
+            min_width = 0,
             -- Width of the code block background:
             --  block: width of the code block
             --  full: full width of the window