[json] Cache JSON schemas locally

[DovydasNavickas]

At the moment I'm travelling and it's not 100% of the time that I have an internet connection.
Turns out that vscode does not cache JSON schemas that are defined in URLs, I presume, here:
https://github.com/Microsoft/vscode/blob/ac8f47b193f08b93e0d05201cf3ecc896914e6a5/extensions/typescript/package.json#L447

It's a pitty that JSON schema for sure did not change since an hour ago, when I had the internet connection, but I have zero completion for tsconfig.json right now, just because I'm offline.

Therefore, it would be super amazing to have JSON schemas cached if the URL did not change.

[aeschli]

duplicate of #11828