feat: enforce lowercase primitive type names

[dsanders11]

The docs are inconsistent in type names (90% string, 10% String, similar for number vs Number, etc). The eventual electron.d.ts output gets them finally converted to string and number, but let's throw an error if the titlecase ones are used in the docs to better enforce consistency and avoid user confusion.

A consequence of this change is a lot of types in electron-api.json are currently String, and they'll all become string - I think that should be fine.