Add annotations to libraries

[donny-dont]

Adds the ability to annotate a Library generated by code_builder. This is to support scenarios like JavaScript interop using package:js where the library itself needs to be annotated with @JS().

[natebosch]

I'm a little hesitant on this one.

Some of our tools allow annotating the first import and treats it as if it was an annotation on the library, but that isn't really the way the grammar works as I understand it. If any tools aren't taking the extra step of reading annotations on the first import this feature will appear misleading.

The slightly more correct way to annotate a library is to have a library statement and a library name. We don't support either in code_builder.

@jakemac53 - WDYT?

If we had dart-lang/language#1073 this would be easy - we could always add a library;.

We could potentially add this feature for now with a comment calling out that support may be inconsistent, and then update it if we get that language feature.

[donny-dont]

Would you be against Library optionally having a name then?

In this case if you add an annotation but not a name then it could throw and then later if the language allows a nameless library declaration and you're targeting a version of Dart with that it removes the restriction.

[jakemac53]

Ya I think we should require a named library from the user if we want to allow annotating it.

[donny-dont]

@jakemac53 @natebosch should I also mixin Library with HasDartDocs and if a name isn't present throw as well while I'm here?

[donny-dont]

Ok I just added the annotations part so I think this is good to review.

[donny-dont]

@natebosch @jakemac53 the library name is now generated. Happy to add documentation as well if that's something that you all want. Wasn't sure since #311 implied documentation might get a revamp.

[jakemac53]

This generally LGTM but I will let Nate have the final say as he is more involved in this package

[donny-dont]

@natebosch any thoughts on this?

[natebosch]

@donny-dont - sorry for the delay. This is published now.