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
