Add JSON schema for manifest.json

[oliverdunk]

It'd be great to provide the editor with a JSON schema to make authoring the manifest.json file easier (with autocomplete etc). I think the Monaco editor does have some support for this.

Unfortunately there isn't an official schema right now but I believe for MV2 at least, there are some floating around.

We would want to do this in a way that allows unknown keys without erroring, and also do some sort of union among all of the browsers so we support all of the different keys.