Add `me` as `guild.me` to repl builtin variables

[ImNimboss]

Rationale

This change is a QoL feature so users have to type less to access the me variable.

Summary of changes made

This adds a new repl builtin variable me, which is a shortcut for the guild.me attribute that represents the bot in a member form.

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 (testing not needed AFAIK)
  • [ ] I have tested these changes against the CI/CD test suite
  • [x] I have updated the documentation to reflect these changes (only the README)
• [ ] 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
• [x] This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • [x] I have proofread my changes for grammar and spelling issues
  • [x] I have tested that any changes regarding Markdown/RST syntax result in a well formed document