Add a Code of Conduct

[CasualPokePlayer]

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This is taken largely from https://github.com/TASVideos/tasvideos/blob/main/CODE_OF_CONDUCT.md

Who to contact for these kinds of issues is still up for discussion. I wasn't sure what to put in here so I only kept in adelikat's email.

Before this is merged, I would like for all currently "active" (i.e. anyone with push access and has committed within the last year) developers to agree to this CoC before merging, and affirming they understand consquences of not following it:

• [x] @adelikat
• [ ] @vadosnaprimer
• [ ] @zeromus
• [ ] @YoshiRulz
• [ ] @Morilli
• [ ] @nattthebear
• [ ] @SuuperW
• [ ] @CasualPokePlayer

A reply like this to this PR will be sufficient:

I, <username>, agree to follow the code of conduct defined in the BizHawk project. I understand violating this agreement will result in disciplinary action and may result in temporary or permanent removal from the BizHawk team.

This PR would be in response to https://github.com/TASEmulators/BizHawk/issues/3172