feat: don't parse self/ctx parameter annotations of prefix command callbacks

[shiftinv]

Summary

get_signature_parameters would previously evaluate annotations of all parameters, including self/ctx, which doesn't allow for code like this:

...
if TYPE_CHECKING:
    from .my_bot import MyBot

class SomeCog(Cog):
    @commands.command()
    async def cmd(self, ctx: Context["MyBot"], stuff: str):
        ...

Admittedly, that pattern may be rare, but due to this issue a workaround was required in #828.
This PR avoids parsing the annotations of the self (if present) and ctx parameter entirely, as they're not relevant for the process.

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 task lint
  • I have type-checked the code by running task 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, ...)

[onerandomusername]

@shiftinv would you please resolve conflicts?

[EQUENOS]

Looks good, tests were successful