Add section & block schema registries for use in the language server and theme checks

[charlespwd]

Describe the solution you'd like
I want folks to be able to easily build other features as though all the schemas of all the files were available at any time in a very performant way.

In checks:

const schema = await context.getSectionSchema('main-product');
const schema = await context.getBlockSchema('slide');

In the language server

const rootUri = await findRootUri(textDocument.uri);
const registry = SchemaRegistryManager.get(rootUri);
const schema = await registry.getSectionSchema('main-product');
const schema = await registry.getBlockSchema('slide');

Under the hood:

• Should only update itself if the file changes (parse only once)
• Should work for unopened files
• Should be lazy (?) tbd...

cc @miazbikowski @albchu