Vim and macvim python3 fix

Fixes for issue #31337:

Vim and MacVim now only *either* support python **or** python3, not both (and, in particular, not the dynamically loaded versions that lead to segfaults in the first place). If either formula is brewed `--with-python3`, python[2] support is automatically disabled. For MacVim, unless `--without-python` is explicitly defined, this shows a Caveat reminding the user that only one version of Python is supported. With Vim, the issue that only one Python version can be supported is made explicit in the list of options available.

Last, while updating the two formulae, I noticed that while the MacVim formula supports Perl "out of the box", Vim's formula does not. As this is confusing, now the Vim formula is adapted to work just as MacVim, with Perl support "on" by default.