Add raw_mention utils

[NeloBlivion]

Summary

Reimplements the various raw_mentions properties from discord.Message under discord.utils. Message.raw_mentions can't be used with regular strings, which is an increasing use case with slash commands.

Also considering a followup commit for automatically converting them (e.g. resolve_mentions?) as counterparts to Message.mentions, Message.channel_mentions and Message.role_mentions but not sure where to implement. Possibly under Guild?

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.