Refactor `GeneralizedFieldSpec` to use a type parameter or `unknown` for `default` type

[kamarmack]

Status quo:

export type GeneralizedFieldSpec = {
	default?: string;
	innerType?: GeneralizedFieldSpec;
	label?: string;
	meta?: GeneralizedFieldSchemaMetadata;
	notOneOf: string[];
	nullable?: boolean;
	oneOf: string[];
	tests: {
		name: 'defined' | 'required';
		params: unknown;
	}[];
	type: 'string' | 'number' | 'boolean' | 'array' | 'mixed';
};

The problem is that default could be any type, so let's use a type parameter or unknown.