fix(vim.fs): avoid fn.fnamemodify in fs.root #37162

Problem: vim.fs.root uses vim.fn.fnamemodify. vim.fn table isn't
available from nvim -ll or thread contexts.

Solution: Swap out vim.fn.fnamemodify for vim.fs.abspath.

This is a temporary workaround and may be reverted since the
long-term plan is to use more fast=true "fn" functions from vim.fs
where possible.

runtime/lua/vim/fs.lua

@@ -450,8 +450,8 @@ function M.root(source, marker)
     })
 
     if #paths ~= 0 then
-      local dir = vim.fs.dirname(paths[1])
-      return dir and vim.fn.fnamemodify(dir, ':p:h') or nil
+      local dir = M.dirname(paths[1])
+      return dir and M.abspath(dir) or nil
     end
   end