metadata api docs, code completion, and validation for all the .xml files

[mshanemc]

Is your feature request related to a problem? Please describe.
I have to hand-edit meta.xml files sometimes. So then I go google the docs to figure out type defs.

Describe the solution you'd like
Code completion, schema validation, and inline documentation for all XML metadata files.

Describe alternatives you've considered
googling by hand. Importing the mdapi as a lang server would also be good (have all the types defined locally instead of opening out to the api docs). That wouldn't give you code samples, but might (if there's an extension somewhere for working with xml) give you linting and autocomplete.

What I want: hovering over compact layout would give me a link to the docs (bonus points if you open the version that matches my file (if defined) or project (if the file I'm in doesn't specify)
https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_compactlayout.htm