fix(.exhaustive): optional discriminant

This PR fixes #278 This new release fixes the following bug in exhaustiveness checking when matching on optional properties: ```ts type Input = { type?: 'one' } | { type: 'two' }; const f1 = (input: Input) => match(input) .with({ type: 'one' }, () => {}) .with({ type: 'two' }, () => {}) .exhaustive(); // shouldn't type-check, but does 👎 const f2 = (input: Input) => match(input) .with({ type: 'one' }, () => {}) .with({ type: 'two' }, () => {}) .with({ type: undefined }, () => {}) // <- the type key needs to be present. .exhaustive(); // shouldn't type-check, but does 👎 ``` These two cases don't type check anymore. They fail with a `NonExhaustiveError<{ type?: undefined; }>`. To fix it, you should do: ```ts type Input = { type?: 'one' } | { type: 'two' }; const f = (input: Input) => match(input) .with({ type: 'one' }, () => {}) .with({ type: 'two' }, () => {}) .with({ type: P.optional(undefined) }, () => {}) // <- the type property may not be there .exhaustive(); // ✅ ``` This is a purely type-level change, the runtime behavior is still the same.