Allow _ and \ to match path separators

[rafeca]

This PR addresses the feedback from @Fedexyz (comment) and @Songworks (comment) around matching full paths using _ or \\.

This is a common pattern in PHP, where in order to create namespaces, people either use underscores or \\ (for real namespacing) on the class names.

Usually, these namespaces match the folder structure, so a class named App_Controllers_MyController or \\App\\Controllers\\MyController will be located in src/App/Controllers/MyController.php.

For convenience, before the fast mode, people used to copy the classname from the editor and paste it on the fuzzy finder, which returned the result src/App/Controllers/MyController.php that the user expected (even if the matching logic was not very clear).

When we implemented the fast mode, this functionality was lost, so this PR adds the relevant logic to fuzzy-native (the library that the fast mode uses to match paths) to add it back.

Alternate designs

I've considered making this mode configurable, but this would add extra complexity on the logic, and since the previous fuzzyfinder matcher didn't have it configurable I thought it's not worth it to add an extra config option.

[nathansobo]

There's a weirdly-worded comment that I noted, but the code looks great. Nice solution. When I looked briefly last week, concerns about exact matches involving _ getting buried gave me pause, but it looks like you've addressed that.