Validate extension contributions against our JSON schemas

[tjcouch-sil]

User Story
As an extension developer, I want to have some level of intellisense and validation on my JSON contribution files so I can write them with confidence.

Description

We have some JSON files that extensions want to provide to contribute to the product in various ways. There are many validations JSON schema cannot do on these that we do in our TypeScript code, but JSON schema gives us validation on at least the overall shape of the data. We should enable extension developers to validate their JSON files against these schemas so they can write them with confidence:

• Move any JSON schema documents that are currently objects within TypeScript files into their own files
• Figure out sharing definitions across these JSON schema files so we can make a base schema file that has some helper definitions like localizeKey and id or stuff like that
• Create a JSON schema for the extension manifest, generate TypeScript types for it, and set up the types so the extension service (and other relevant files) uses those types instead of the hand-written TypeScript ones
• Set up the manifest.json to receive schema validation in VSCode in all relevant repositories (core, both templates, etc). Also potentially set up any other .json contribution docs we have in those repos to receive validation as well. Or maybe make some guesses as to what those would potentially be called. Like menu contributions could potentially be in assets/menus.json or something. Not sure this is worth doing.
• Explain in appropriate readmes (template for example) about these JSON schemas and how to set up .json contribution documents to be validated on them