loaders: add matching loader

[blaggacao]

No description provided.

[figsoda]

pushed some changes to make this fit kind of what I'm envisioning (I haven't updated the docs), tell me what you think

• renamed match -> matches: I feel like match might be confused with builtins.match. There are some remaining uses of matches in the code base that means something else, but for the public facing docs this should be pretty unambiguous
• removed __functor, matchers are now called with matcher.loader instead: I feel like this would be easier to understand
• reimplemented parsePath: the previous implementation uses builtins.match and fails on file names that don't have a ., I don't think this is possible with builtins.match since nix uses POSIX regex and there are is no laziness (.+?). Also moved it into its own file since it's getting long
• renamed nixFiles -> nix: I like it shorter since we are probably adding stuff like matchers.json and matchers.toml in the future. If this is too ambiguous maybe we can rename regex to withRegex

I'm still a bit unsure about whether matches should take the file name or the file name without prefixing _s (called stripped in parsePath's implementation). I do like this more than taking the full path

[blaggacao]

I'm still a bit unsure about whether matches should take the file name or the file name without prefixing _s (called stripped in parsePath's implementation). I do like this more than taking the full path.

Agreed, and so to the rest of the changes. I understood that the matching groups are only available to the regex interface and that's much better this way, going to update the docs, then and we should be ready to go.