docs: rename builtin.txt, eval.txt #34212

Problem:
Despite the name, `builtin.txt` only includes Vimscript functions, which
is confusing, especially to people who only use Lua to configure Nvim

Solution: From justinmk's suggestion
- Rename `builtin.txt` to `vimfn.txt`
- Rename `eval.txt` to `vimeval.txt`
- The tags `*builtin.txt*` and `*eval.txt*` should be kept for Vim-patches

Closes #33743

src/gen/gen_help_html.lua

@@ -94,7 +94,7 @@ local redirects = {
 
 -- TODO: These known invalid |links| require an update to the relevant docs.
 local exclude_invalid = {
-  ["'string'"] = 'eval.txt',
+  ["'string'"] = 'vimeval.txt',
   Query = 'treesitter.txt',
   matchit = 'vim_diff.txt',
   ['set!'] = 'treesitter.txt',