feat(runtime): Lua ftplugin 'includeexpr' #32719

Problem:
Current `'includeexpr'` in runtime/ftplugin/lua.vim doesn't work with Nvim Lua.

Solution:
Provide an improved 'includeexpr' for Lua in "ftplugin/lua.lua".

Closes: https://github.com/neovim/neovim/issues/32490

runtime/lua/vim/_ftplugin/lua.lua

@@ -0,0 +1,26 @@
+local M = {}
+
+--- @param module string
+---@return string
+function M.includeexpr(module)
+  ---@param fname string
+  ---@return boolean
+  local function filereadable(fname)
+    return vim.fn.filereadable(fname) == 1
+  end
+
+  local fname = module:gsub('%.', '/')
+
+  local root = vim.fs.root(vim.api.nvim_buf_get_name(0), 'lua') or vim.fn.getcwd()
+  for _, suf in ipairs { '.lua', '/init.lua' } do
+    local path = vim.fs.joinpath(root, 'lua', fname .. suf)
+    if filereadable(path) then
+      return path
+    end
+  end
+
+  local modInfo = vim.loader.find(module)[1]
+  return modInfo and modInfo.modpath or module
+end
+
+return M