[feature] Federate status language in and out

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request implements contentMap handling to append language to outgoing statuses, and parse language from incoming statuses (see the new docs entry for how this is done exactly).

Since go-fed handles contentMap and content a bit funkily, some new normalization logic has also been added to ensure that content and contentMap are appended separately to outgoing statuses, and to ensure that they're parsed separately and then recombined on incoming statuses.

Also added some tests, and made a few consistency fixups on the normalization / serialization functions in the ap package.

closes #1277

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

Just a few comments remaining! Otherwise this is looking really good 👀