refactor: unify slash/prefix command signature evaluation

[shiftinv]

Summary

This PR makes slash command signature evaluation use the existing prefix command mechanisms (notably, utils.evaluate_annotation), instead of relying on typing.get_type_hints which has historically been somewhat inconsistent.

There aren't any immediate issues with the previous code; this change is in preparation for Python 3.12 adjustments in the near future. Right now, this shouldn't have any effect in practice.
It may look complicated, but it's really just moving around different parts of the code to new locations and dropping commands.params.signature() in favour of utils.get_signature_parameters().

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[EQUENOS]

Just several nits. Looks good overall, I especially liked that you moved some parsers to utils.py, this way they can be easily used by ext-packages.

[EQUENOS]

Lgtm