refactor(help): move local-additions to Lua #37831

Problem:
- ~200 line function of hard-to-maintain C code.
- Local Addition section looks messy because of the varying description
  formats.

Solution:
- Move code to Lua.
- Have a best-effort approach where short descriptions are right
  aligned, giving a cleaner look. Long descriptions are untouched.

runtime/doc/help.txt

@@ -143,7 +143,7 @@ Standard plugins ~
 See |standard-plugin-list|.
 
 Local additions ~
-							*local-additions*
+							     *local-additions*
 
 ------------------------------------------------------------------------------
 Bars example								*bars*

runtime/lua/vim/_core/help.lua

@@ -122,10 +122,77 @@ function M.escape_subject(word)
     -- E.g. '`command`,' --> 'command' (backticks are removed too, but '``' stays '``')
     word = word:gsub([[^'([^']*)'.*]], [['%1']])
     word = word:gsub([[^{([^}]*)}.*]], '{%1}')
-    word = word:gsub([[^`([^`]+)`.*]], '%1')
+    word = word:gsub([[.*`([^`]+)`.*]], '%1')
   end
 
   return word
 end
 
+---Populates the |local-additions| section of a help buffer with references to locally-installed
+---help files. These are help files outside of $VIMRUNTIME (typically from plugins) whose first
+---line contains a tag (e.g. *plugin-name.txt*) and a short description.
+---
+---For each help file found in 'runtimepath', the first line is extracted and added to the buffer
+---as a reference (converting '*tag*' to '|tag|'). If a translated version of a help file exists
+---in the same language as the current buffer (e.g. 'plugin.nlx' alongside 'plugin.txt'), the
+---translated version is preferred over the '.txt' file.
+function M.local_additions()
+  local buf = vim.api.nvim_get_current_buf()
+  local bufname = vim.fs.basename(vim.api.nvim_buf_get_name(buf))
+
+  -- "help.txt" or "help.??x" where ?? is a language code, see |help-translated|.
+  local lang = bufname:match('^help%.(%a%a)x$')
+  if bufname ~= 'help.txt' and not lang then
+    return
+  end
+
+  -- Find local help files
+  ---@type table<string, string>
+  local plugins = {}
+  local pattern = lang and ('doc/*.{txt,%sx}'):format(lang) or 'doc/*.txt'
+  for _, docpath in ipairs(vim.api.nvim_get_runtime_file(pattern, true)) do
+    if not vim.fs.relpath(vim.env.VIMRUNTIME, docpath) then
+      -- '/path/to/doc/plugin.txt' --> 'plugin'
+      local plugname = vim.fs.basename(docpath):sub(1, -5)
+      -- prefer language-specific files over .txt
+      if not plugins[plugname] or vim.endswith(plugins[plugname], '.txt') then
+        plugins[plugname] = docpath
+      end
+    end
+  end
+
+  -- Format plugin list lines
+  -- Default to 78 if 'textwidth' is not set (e.g. in sandbox)
+  local textwidth = math.max(vim.bo[buf].textwidth, 78)
+  local lines = {}
+  for _, path in vim.spairs(plugins) do
+    local fp = io.open(path, 'r')
+    if fp then
+      local tagline = fp:read('*l') or ''
+      fp:close()
+      ---@type string, string
+      local plugname, desc = tagline:match('^%*([^*]+)%*%s*(.*)$')
+      if plugname and desc then
+        -- left-align taglink and right-align description by inserting spaces in between
+        local plug_width = vim.fn.strdisplaywidth(plugname)
+        local desc_width = vim.fn.strdisplaywidth(desc)
+        -- max(l, 1) forces at least one space for if the description is too long
+        local spaces = string.rep(' ', math.max(textwidth - desc_width - plug_width - 2, 1))
+        local fmt = string.format('|%s|%s%s', plugname, spaces, desc)
+        table.insert(lines, fmt)
+      end
+    end
+  end
+
+  -- Add plugin list to local-additions section
+  for linenr, line in ipairs(vim.api.nvim_buf_get_lines(buf, 0, -1, false)) do
+    if line:find('*local-additions*', 1, true) then
+      vim._with({ buf = buf, bo = { modifiable = true, readonly = false } }, function()
+        vim.api.nvim_buf_set_lines(buf, linenr, linenr, true, lines)
+      end)
+      break
+    end
+  end
+end
+
 return M