feat: support copy/pasting tag link nodes in rich text mode

[aalear]

This PR addresses tag link serialization for #56.

The core of the change is switching from NodeView to a schema definition with appropriate toDOM/parseDOM implementations.

It looks like we never wrote tests for the original NodeView nor do we have existing tests for other schema specifications. I'm happy to take a stab at adding some, but for now I left them out.

Also note that pasting from a final rendered document is not currently supported. Part of the issue is that public Q&A still uses .post-tag rather than .s-tag from Stacks. I'm not sure if it's worth adding .post-tag checks into the editor repo. Feels like we should be moving towards making the Q&A rendering use Stacks classes instead, although that's also a non-trivial undertaking.

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[aalear]

@b-kelly I re-tested this the other day and I think everything is good to go. No feature-related changes needed after #158 went in.