wip: strictly types fields

[jacobsfletch]

Resolves #34 by strictly typing fields.

HELP NEEDED

I can't seem to get these stricter types to resolve in my intersection. Basically I have an object with known properties of Type A and unknown properties of Type B. Here's a very simple breakdown of my exact issue:

export type BaseFields = {
  text: boolean | Partial<Field>
} & {
  [key: string]:
    Block
    | ((fieldConfig?: FieldConfig) => Block)
}

const baseFields: BaseFields = {
  text: true,
}

// Produces this error
// Property 'text' is incompatible with index signature.
//       Type 'boolean' is not assignable to type 'Block | ((fieldConfig?: FieldConfig) => Block)'.

// Note: Using `Exclude` on the union type also does not work, i.e. [key: Exclude<string, 'text'>]: Block | (fieldConfig?: FieldConfig) => Block

Here's the exact errors I'm running into with this code:

I found a related GitHub discussion here: microsoft/TypeScript#20597

[IAmNatch]

Seems to be impossible at this time without either:
a) runtime checks
b) adding the text union value to the block union (even though its wrong), thus being a subset of the main type instead of a conflict.