[ffigen] Add initial C++ scope support

[jakobkordez]

Description

This PR adds support for scoped (for now only) enums. That is enums declared within namespaces, classes or structs.

Related Issues

None

PR Checklist

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've run dart tool/ci.dart --all locally and resolved all issues identified. This ensures the PR is formatted, has no lint errors, and ran all code generators. This applies to the packages part of the toplevel pubspec.yaml workspace.
• All existing and new tests are passing. I added new tests to check the change I am making.
• The PR is actually solving the issue. PRs that don't solve the issue will be closed. Please be respectful of the maintainers' time. If it's not clear what the issue is, feel free to ask questions on the GitHub issue before submitting a PR.
• I have updated CHANGELOG.md for the relevant packages. (Not needed for small changes such as doc typos).
• I have updated the pubspec package version if necessary.

[jakobkordez]

@liamappelbe Mind looking at this and giving your opinion before I proceed with full support?

[liamappelbe]

@jakobkordez Thanks for the PR, but C++ in FFIgen is a GSoC project. That is, it's a special project being worked on by a student at the moment. So we can't really accept external contributions to it currently. If you're still interested after GSoC is finished (in a few months) we'd be happy to accept PRs at that time.

If you want something to work on in the meantime, you can take a look at the good-first-issues. Btw, a lot of those may appear to be claimed by other external devs, but afaik they've all gone dormant, so feel free to grab whatever looks interesting to you.