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

(cherry picked from commit 9d5eb3eda5)

test/old/testdir/test_help.vim

@@ -76,7 +76,7 @@ endfunc
 
 func Test_help_expr()
   help expr-!~?
-  call assert_equal('eval.txt', expand('%:t'))
+  call assert_equal('vimeval.txt', expand('%:t'))
   close
 endfunc