Add type safety to fieldRefs

[beaucollins]

fieldRefs is more accurately represented as a const Record type. By as consting the strict key and value types are captured by the type system.

Before, it's an array of objects that get searched through

After, it's a Record of explicit keys and ref values:

We don't need getFieldRef anymore, the keys are statically known. And no more iterating just to look up the correct ref.

[stevekuznetsov]

Love it. I wish we could decorate the actual form context with the refs since we're not enforcing that the keys here match anything in the form, but /shrug. I know you saw some of the other TODOs around typing in here - we originally wrote this without strict tsconfig on, which let us be a little loosey-goosey, and adding types after the fact was tricky since I'm not an expert in TypeScript magic.