WIP: Add STABLE_PACKAGE_NO_IMPORT_UNSTABLE lint rule

[bufdev]

I'm not yet sure if we want to add this into mainline. Opening this for discussion.

This is re: https://github.com/bufbuild/buf/issues/2986.

This adds a STABLE_PACKAGE_NO_IMPORT_UNSTABLE lint rule, with no category, that will produce a lint failure if an stable package imports a file with an unstable package. For example:

1. If a.proto imports b.proto, and a.proto has package a.v1:

• If b.proto has package foo, no failure - cannot determine package stability.
• If b.proto has package foo.v1, no failure - stable imports stable.
• If b.proto has package foo.v1beta1, failure - stable imports unstable.

2. If a.proto has package a:

• No failure ever - cannot determine package stability.

3. If a.proto has package a.v1beta1:

• No failure ever - package is unstable, does not matter what it imports.

If we do decide to go ahead with this, this PR:

• Needs some discussion as to the rule name.
• Needs testing.