Rework hybrid commands

[Ableytner]

This PR reworks hybrid commands to allow for more flexibility when using string arguments in commands.

Breaking change: defining a string argument as

async def my_command(self, ctx, *my_arg: list[str]):
    my_arg_combined = " ".join(["".join(item) for item in my_arg])

is no longer allowed, please use

async def my_command(self, ctx, my_arg: str):
    pass

instead. The functionality will be the same. This also supports string inputs with whitespaces.