Fixes#20694
Implements super basic new platform `core` system, which includes two services: `core.injectedMetadata` and `core.legacyPlatform`. The `core` currently has two responsibilities:
1. read the metadata from the DOM and initialize the `ui/metadata` module with legacy metadata, proving out how we plan to expose data from the new platform through the existing APIs/modules to the legacy platform.
2. bootstrap the legacy platform by loading either `ui/chrome` or `ui/test_harness`
Because `core` mutates the `ui/metadata` module before bootstrapping the legacy platform all existing consumers of `ui/metadata` won't be impacted by the fact that metadata loading was moved into the new platform. We plan to do this for many other services that will need to exist in both the legacy and new platforms, like `ui/chrome` (see #20696).
