Better Typescript validation on errorMessages

[brophdawg11]

errorMessages keys should ideally be restricted to ValidityStateKey unioned with keys in customValidations.

let formDefinitions: FormSchema = {
  inputs: {
    name: {
      validationAttrs: { 
        required: true,
      },
    },
    custom: {
      customValidations: {
        isThisThingValid() { ... }
      },
      errorMessages: {
        // ✅ OK since this matches a customValidation
        isThisThingValid: () => { ... }
        // ❌ Should fail because nope is not a ValidityStateKey or a customValidation name
        nope: () => { ... }
      },
    },
  },
  errorMessages: {
    // ✅ OK since this matches a ValidityStateKey
    valueMissing: () => { ... }
    // ❌ Should fail because nope is not a ValidityStateKey or a custom validation name
    nope: () => { ... }
  }
}