fix: Don't forget branding in value type of Record.

[untio11]

Before this fix, branding information was lost design time on the key and value types of a Record type:

type BrandedString = The<typeof BrandedString>;
const BrandedString = string.withBrand('BrandedString');

type BrandedKVRecord = The<typeof BrandedKVRecord>;
const BrandedKVRecord = record(BrandedString, BrandedString);

// Old behaviour:
expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, string>>();

// New behaviour:
expectTypeOf<BrandedKVRecord>().toEqualTypeOf<Record<string, BrandedString>>();

This same helper type (Unwidened<T>) can be used on the key type too, however, this breaks compatibility with .literal() because DeepUnbranded<T> can't really deal with branded key types yet.