Runtime Tag providers for workspace package.json

[vknabel]

Hi thanks for all your work!

Currently dependencies inside the node_modules folder may declare custom tags and attributes within their package.json's vetur-attribute. See https://github.com/vuejs/vetur/blob/master/docs/framework.md#custom-tags--attributes

This PR extends this feature to also support the package.json directly inside the workspace path.
This is especially useful, for global components and dependencies not declaring vetur tags.

[octref]

Sorry, superseded by #2018 and already published. You didn't link to #1364 so I didn't realize that there's an existing PR. Still, thanks a lot.

[vknabel]

Ah, I didn’t search for existing issues, just for PRs. So don’t worry #2018 also updated the docs (which is awesome). 👍