feat: `..` in tuple pattern

[asterite]

Description

Problem

No issue, but:

• with this we could move forward with #8420 without ever introducing a breaking change (we can add , .. in the existing code where the new tuple element will show up before merging that PR)
• I guess it's something we were going to eventually implement, so...

Summary

I decided to implement this to unblock #8420 because I thought it was going to be easy: "just" parse the new syntax, then ignore the extra elements. But in the end it was a bit more complex:

• I initially thought that ".." could only show up at the end of a tuple pattern, but in reality it can show up in any part of it
• The ".." logic must also be applied during monomorphization in a couple of places, and for the interpreter

That said, besides that it was relatively straight-forward.

I included a big test that proves this is working well in many cases.

Additional Context

• There was a pattern_to_string function that replicated the logic of turning a pattern into a string, so I removed it
• I took the opportunity to extract some code into helper functions to reduce the size of some big functions
• I noticed some methods were unused so I removed them

Documentation

Check one:

• [ ] No documentation needed.
• [x] Documentation included in this PR.
• [ ] [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• [x] I have tested the changes locally.
• [x] I have formatted the changes with Prettier and/or cargo fmt on default settings.

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