core(tsc): use Config class to define Config type

[brendankenny]

Previously there was the Config class (in config.js) and the LH.Config type (in config.d.ts). The LH.Config type implemented LH.Config.Json, ensuring that any LH.Config was also a valid LH.Config.Json.

When passing around an instance of Config we called it an LH.Config, which meant the compiler was enforcing that it implemented both interfaces, but that means two sources of truth for what a "Config" is and indirect errors if those interfaces were violated.

Based on @patrickhulce's idea in #5451 to enforce interfaces until tsc supports @implements tags, we can eliminate the d.ts declaration of LH.Config and just use the Config class directly, just asserting that the object is a valid LH.Config.Json near the end of the constructor in a throwaway variable.

(the rest of the config interfaces staying separate for now because @typedef doesn't support inheritance yet 🙄 and they aren't clearly defined anywhere with a constructor or single object literal)

[brendankenny]

non-functional for now

[patrickhulce]

LGTM!