Add support for the message_content intent

[BobDotCom]

Rationale

Privileged intents are listed in the root command, and this adds support for detection of the "message_content" intent, if available. Contrary to #142, this uses the specific message content intent (unlike the messages intent it only applies to content, hence the name - and is also privileged). This intent is not supported by discord.py, but as it's a name declared by the API, it can be considered a "standard" name and should be consistent across forks (A "Smaller feature-detection based change" as defined here). If this feature does not detect support for the message_content intent, it will revert to the old functionality (simply not displaying info on this intent).

I can personally verify that this works with Pycord (already on API v10), and should work with disnake when their v10 support PR is merged. The same should be true for edpy, when their v10 PR is merged, and nextcord as well - when this PR is merged.

Summary of changes made

Pretty straightforward, simply adds the message_content intent to the root command. At first I thought this would be useless, but bots can still receive message content without the intent in some cases, so the command can be invoked even if the intent is off.

Checklist

• [x] This PR changes the jishaku module/cog codebase
  • [x] These changes add new functionality to the module/cog
  • [ ] These changes fix an issue or bug in the module/cog
  • [x] I have tested that these changes work on a production bot codebase
  • [x] I have tested these changes against the CI/CD test suite
  • [ ] I have updated the documentation to reflect these changes
• [ ] This PR changes the CI/CD test suite
  • [ ] I have tested my suite changes are well-formed (all tests can be discovered)
  • [ ] These changes adjust existing test cases
  • [ ] These changes add new test cases
• [ ] This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • [ ] I have proofread my changes for grammar and spelling issues
  • [ ] I have tested that any changes regarding Markdown/RST syntax result in a well formed document

[BobDotCom]

Unsure why the AppVeyor CI failed. It says invalid Slack webhook, so I assume it's unrelated to my code.

[ooliver1]

Would be nice if we got a comment on that