Merge ffigen into dart-lang/native

[dcharkes]

The process of merging a (non-mono) repo into dart-lang/native can be found here: dart-lang/tools#100. @liamappelbe could you follow these steps for the FFIgen repo and make a PR in the same style?

Originally posted by @dcharkes in https://github.com/dart-lang/ffigen/issues/612#issuecomment-1788516342

Filing a separate issue to track this.

[dcharkes]

@liamappelbe I've hit merge on dart-lang/native. 🎉

Please follow up with:

• Push tags to dart-lang/native.
• Follow up with a PR adding links to the top-level readme on dart-lang/native.
• Add a PR to https://github.com/dart-lang/ffigen/ with it's readme pointing to the new location.
• Migrate all issues and labels to dart-lang/native.
• Archive https://github.com/dart-lang/ffigen/ (if you have the rights to do so).

[liamappelbe]

All issues have been transferred, and https://github.com/dart-lang/ffigen/ is archived