docs: apply current colorscheme to default highlight groups

Problem: Not all default highlight groups show their actual colors.
Solution: Refactor `vimhelp.lua` and apply it to all relevant lists (UI
groups, syntax groups, treesitter groups, LSP groups, diagnostic groups).

runtime/lua/vim/vimhelp.lua

@@ -2,26 +2,28 @@
 
 local M = {}
 
--- Called when editing the doc/syntax.txt file
-function M.highlight_groups()
-  local save_cursor = vim.fn.getcurpos()
-
-  local start_lnum = vim.fn.search([[\*highlight-groups\*]], 'c')
-  if start_lnum == 0 then
-    return
-  end
-  local end_lnum = vim.fn.search('^======')
-  if end_lnum == 0 then
-    return
-  end
-
+--- Apply current colorscheme to lists of default highlight groups
+---
+--- Note: {patterns} is assumed to be sorted by occurrence in the file.
+--- @param patterns {start:string,stop:string,match:string}[]
+function M.highlight_groups(patterns)
   local ns = vim.api.nvim_create_namespace('vimhelp')
   vim.api.nvim_buf_clear_namespace(0, ns, 0, -1)
 
-  for lnum = start_lnum, end_lnum do
-    local word = vim.api.nvim_buf_get_lines(0, lnum - 1, lnum, true)[1]:match('^(%w+)\t')
-    if vim.fn.hlexists(word) ~= 0 then
-      vim.api.nvim_buf_set_extmark(0, ns, lnum - 1, 0, { end_col = #word, hl_group = word })
+  local save_cursor = vim.fn.getcurpos()
+
+  for _, pat in pairs(patterns) do
+    local start_lnum = vim.fn.search(pat.start, 'c')
+    local end_lnum = vim.fn.search(pat.stop)
+    if start_lnum == 0 or end_lnum == 0 then
+      break
+    end
+
+    for lnum = start_lnum, end_lnum do
+      local word = vim.api.nvim_buf_get_lines(0, lnum - 1, lnum, true)[1]:match(pat.match)
+      if vim.fn.hlexists(word) ~= 0 then
+        vim.api.nvim_buf_set_extmark(0, ns, lnum - 1, 0, { end_col = #word, hl_group = word })
+      end
     end
   end