fix: add option to specify picker layout for snacks picker used in Opencode

[MrBanh]

The default snacks picker layout is used for various sources that is not always ideal for this plugin's use case. E.g., many layouts have preview = 'file', which is useless for the session_picker. There's no way to disable preview just for Opencode's snacks picker without affecting other uses of snacks picker for other sources. Looks like it's because of this change

For example,

{
  "folke/snacks.nvim",
  opts = {
    picker = {
      layout = { preset = "ivy" }
    }
  }
}

The ivy preset has a preview window that doesn't work with session_picker, but I don't want to disable it because the default picker layout is used for a bunch of builtin sources where I rely on the file preview.

This change allows users to specify a layout in the config. I only added the option for snacks, but we can extend it to the other pickers as well.

There's 3 main ways on how to change the snacks picker layout

1. Don't specify the new options -> it'll just default to the user's snack picker layout preset

2. Specify the new options for opencode, e.g.

   require("opencode").setup({
     ui = {
       picker = {
         snacks_layout = {
           layout = { border = "none", box = "vertical", ... }
         },
       },
     },
   })

3. Specify a preset available for snacks picker OR a custom layout

   -- opencode.lua
   require("opencode").setup({
     ui = {
       picker = {
         snacks_layout = {
           preset = "custom_layout" -- or builtin snacks, like "select", "default", etc
         },
       },
     },
   })    

   -- snacks.lua
   { 
     "folke/snacks.nvim",
     opts = {
       picker = {  
         layouts = {  
           custom_layout  = {
             layout = { border = "none", box = "vertical", ... }
             -- ...
   }

[sudo-tee]

This is a good idea, the implementation seem reasonable also

Thanks for the PR