Add a warning infrastructure

[thufschmitt]

Is your feature request related to a problem? Please describe.

We have a bunch of behaviours (mostly in the evaluator, but probably not only) that we'd like to get rid of, but can't because of backwards-compatibility.

A good middle-ground solution is to keep them, but add a warning to prevent people from accidentally relying on them.
We currently never do that because doing so would require adding a way to opt-out easily, and

1. We don't really want to do it in a, ad-hoc fashion
2. We don't have infrastructure to do it in a principled fashion.

Describe the solution you'd like

Some infrastructure (similar to the experimental-features one?) allowing to declare warnings and expose a way to easily toggle them (or turn into hard errors) from the CLI or the configuration files.

Describe alternatives you've considered

• Allow breaking changes to the language (with something like [RFC 0137] Nix language versioning rfcs#137)
• Add warnings in a more ad-hoc way (just call warn) in a bunch of places

Additional context

Random sampling of things that we might want to get warning for:

• unexpected result due to paths stripping trailing / #10262
• no-absolute-path-literals experimental feature #8738
• Remove the no-url-literals experimental feature #10048
• Make sure we can fix the parser's bad list/number interactions #8613
• Support hexadecimal and octal syntax for integers #7578
• __curPos keyword is undocumented #10036

Priorities

Add 👍 to issues you find important.

[piegamesde]

On the Lix side, I just copied the experimental features code 1:1 into "deprecated features": https://git.lix.systems/lix-project/lix/src/branch/main/src/libutil/deprecated-features.cc

It's not perfect by any means but it's a design as good as any other, so Nix may as well copy it and keep configuration compatibility