Improve discriminator type

[sschw]

Right now the type of a discriminator is a string or string union.

This is working but not actually representing the correct type. The type should be the same as the field that the discriminator is used on. Meaning if it is an enum, the discriminator should also be of said enum type instead of a string union.

Therefore we should use the discriminator type and extract the allowed keys of the type.

It seems to be, this type should cover this use case:

(Omit<PropName, 'descriminatorType'> & {
  descriminatorType: Extract<PropName['descriminatorType'], 'decriminator'>;
});