Adds ability to register custom tag providers

[eamodio]

This will hopefully allow other extensions to extend the html parsing with their own tags -- for example https://github.com/aurelia/vscode-extension

[aeschli]

The API makes sense, but note that in the case of VSCode the service is used on HTML language server that runs in its own process.
We have no mechanism yet on how to contribute to the server.

[eamodio]

Ah - right. But at least this would allow other extensions to pull in this lib directly, rather than needing to re-implement.

[octref]

@eamodio Would #41 work for you? You can read microsoft/vscode#62976 for more context.

[octref]

Closing this as we probably won't go with this approach. JSON contributions make more sense. We'll likely introduce contributes.html.tags similar to how we allow markdown contributions: https://code.visualstudio.com/docs/extensionAPI/api-markdown.

[eamodio]

@octref I think so, but honestly I can't really remember the full context as it was a long time ago. 😁