feat: implement positional flags

[Vioshim]

Summary

This allows FlagConverters to be used after the first text expression, for example

class HandleText(FlagConverter, prefix="--", delimiter=" "):
    text: str = commands.flag(positional=True)
    flip: bool = False
    
@bot.bridge_command()
async def welcome(ctx: bridge.BridgeContext, *, flags: HandleText):
    text = flags.text[::-1] if flags.flip else flags.text
    await ctx.respond(text)

# ?welcome Hello world
# ?welcome Hello world --flip True
# ?welcome --text Hello world
# ?welcome --text Hello world --flip True

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[Lulalaby]

@Vioshim please work on dorus comment and resolve conflcts

[Paillat-dev]

I have no idea how this works, but if the only thing stopping it from getting merged are conflicts, I can sure resolve those.

[Lulalaby]

@Pycord-Development/contributors can we get some testing here :D

[Paillat-dev]

File "C:\Users\Jérémie\Documents\GitHub\pycord\thing.py", line 23, in <module>
    class BasicText(FlagConverter, prefix="--", delimiter=" "):
  File "C:\Users\Jérémie\Documents\GitHub\pycord\thing.py", line 24, in BasicText
    text: str = flag(positional=True)
                ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Jérémie\Documents\GitHub\pycord\discord\ext\commands\flags.py", line 143, in flag
    return Flag(
           ^^^^^
TypeError: Flag.__init__() got an unexpected keyword argument 'name'

The error does not seem to be caused by this PR, but rather by me using python 3.11. The error is not present in my testing when using python 3.10.

fix in #2759

Other than that, this seems to work to me, although I don't think I'm experienced enough with flags - and prefixed commands to assess with good certainty that everything works as expected.

[Lumabots]

does this need testing or is it ready to merge ?

[Paillat-dev]

Needs testing + doesn't work with python 3.11+