feat: adds support for schema store

[Vincent Biret (baywet)]

this pull request adds support for json schema store so schemas are suggested automatically based on https://www.schemastore.org/ when no $schema is present

[Martin Aeschlimann (aeschli)]

Schema store loading and management should not be part of the JSON language service.

• we don't want to hard-code any schema registry URL, nor it's file format
• the requestService (type SchemaRequestService) is spec to load schemas, not catalogs. It's API and implemented by users of the service. VS Code is only one of them
• there are already APIs to pass schema associations via the LanguageSettings. I would first try to get it working with these. If that doesn't work, add a (optional) property LanguageServiceParams.readFaillbackSchema: (resource: string) => string | undefined

[Vincent Biret (baywet)]

Martin Aeschlimann (@aeschli) thanks for the feedback. I've moved the code that resolves the catalog, etc... to the extension itself.
Let me know if you have any additional comments or questions.

[Vincent Biret (baywet)]

superseded by microsoft/vscode#327628