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)

scripts/vim-patch.sh

@@ -293,6 +293,10 @@ preprocess_patch() {
   LC_ALL=C sed -Ee 's/( [ab]\/src\/nvim)\/option\.h/\1\/option_vars.h/g' \
     "$file" > "$file".tmp && mv "$file".tmp "$file"
 
+  # Rename runtime/doc/eval.txt to runtime/doc/vimeval.txt
+  LC_ALL=C sed -Ee 's/( [ab]\/runtime\/doc)\/eval\.txt/\1\/vimeval.txt/g' \
+    "$file" > "$file".tmp && mv "$file".tmp "$file"
+
   # Rename version*.txt to news.txt
   LC_ALL=C sed -Ee 's/( [ab]\/runtime\/doc)\/version[0-9]+\.txt/\1\/news.txt/g' \
     "$file" > "$file".tmp && mv "$file".tmp "$file"