Convert between perl and vim regex

Searches in notgrep are usually using Perl Compatible Regular
Expressions. Convert these to vim-style as best I can.

In the NotGrepFromSearch case, we know our original query, so search for
that instead.

Considered added some known atoms from |perl-patterns|, but I'll leave
disabled until I figure out if this is a good idea.
Do not include case atoms

I don't use this often enough for it to be useful so it's more likely
to cause a false positive.