Describe the returned object of chrome.runtime.getManifest

[`@types/chrome` does it explicitly](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/66ace08532d370ac54f80bc5a95f0e8902fbefb0/types/chrome/index.d.ts#L7119-L7325) while you in this repo might be able to auto-generate it from source e.g. by walking idl/json files referenced in [api_sources.gni](https://crsrc.org/chrome/common/extensions/api/api_sources.gni). If not feasible, just copy it from `@types/chrome` (maybe even automatically via a plain regexp or smart parsing with acorn).