fix: unescape escaped regex elements

[cbachhuber]

Otherwise, the hook raises paths like foo/bar\.txt as nonexistent, but this is accepted by pre-commit/prek.

[cbachhuber]

This is basically the solution from https://mentaljetsam.wordpress.com/2007/04/13/unescape-a-python-escaped-string/ for unescaping

[hofbi]

So far I always used this as a chance to cleanup. Without the escape it is much more readable.

[cbachhuber]

That's fair. Such slightly overly general regexes never hurt us. Nobody will accidentally create foo?txt instead of foo.txt. Other hooks would probably fail as well.

Still, this behavior is implicit and users wonder why this hook supports less than pre-commit/prek does. In the words of copilot:

pre-commit/prek applies exclude patterns as Python regexes via re.search() against file paths. In Python regex, \. matches a literal dot — so at1\.dbc correctly excluded at1.dbc, maven_install\.json correctly excluded maven_install.json, etc.

The problem was only in the meta-hook (check-non-existing-and-duplicate-excludes), which has its own naive path-extraction logic that:

1. Strips (?x)^(, removes ALL ), splits on |
2. Takes remaining fragments and checks them as literal filesystem paths with Path(entry).resolve().exists()

Should we make this more obvious, for example by documenting?

[cbachhuber]

We'll continue with this in #146