increased type safety via string literals

[dgreene1]

We would get considerably better type safety out of many of the form instance functions if we used the keyof operator. For instance, this simple change would make it impossible to accidentally mispell the field that we're looking for.

export interface FormInstance<Values = any> {
    getFieldValue: (name: NamePath) => StoreValue;

to this:

export interface FormInstance<Values = any> {
    getFieldValue: <NamePath extends keyof FormFieldsAndValues>(name: NamePath) => FormFieldsAndValues[NamePath] | undefined;

[dgreene1]

If approved, I will submit this PR.