Support app command contexts

[RedKinda]

Summary

Adds @dm_only and @private_only decorators to allow specifying the new contexts field in app commands, as specified here, as well as the relevant init args.

The docs mention that if this field is specified, the permission calculation ignores the dm_permission field. Not sure how this should be reflected in the library for backwards compatibility purposes.

Checklist

• If code changes were made then they have been tested. (needs a bit more testing for groups and context commands)
  • I have updated the documentation to reflect the changes.
• 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, ...)

[Rapptz]

Thanks for the PR.

Just some minor bikeshedding/comments for the first pass:

1. AppCommandContext is not an enum, it's actually a flag type. Discord has recently been doing bitfields using an array because they falsely believe it leads to better developer experience since they are targeting developers that do not know how to bitshift. I suggest migrating this over to use the new ArrayFlags type and work with it. I have tried to get them to change this to a regular integer but they seem to want to not want to change it to the more space-efficient and sane format.
2. allowed_contexts and any external reference to context in this well context should probably be called restrict instead. The type should still be called AppCommandContext but the attribute (and maybe eventual decorator) should be called restrict.
3. private_only should probably be renamed to private_channel_only for consistency with the library (e.g. abc.PrivateChannel).
4. The flag names for AppCommandContext should not use the Discord naming scheme since they are bad, instead it should probably be guild, dm_channel, and private_channel.

[Rapptz]

You're missing the documentation for the new decorators. They need to be added to the API reference. Other than that this PR looks good.

[Gobot1234]

This should either drop the ability to optionally use parens or add the necessary overloads here otherwise you'll get type errors.

[Zalk0]

Suggested change

	@app_commands.allow_contexts(guilds=False, dms=False, private_channels=True)
	@app_commands.allow_contexts(guilds=True, dms=False, private_channels=True)

Didn't you want to set guilds to True in the example as that's what you say in the response message just below

[Rapptz]

Superseded by #9760