fix: exhaustive checking with nested P.nonNullable patterns

[gvergnaud]

Fixes issue #247

The when combining P.nonNullable and P.nullish, you should get an exhaustive pattern matching expression, but the following case was incorrectly considered non-exhaustive:

declare const input: {
  nested: string | number | null | undefined;
};

const res = match(input)
  .with({ nested: P.nonNullable }, (x) => {/* ... */})
  .with({ nested: P.nullish }, (x) => {/* ... */})
  // should type-check
  .exhaustive();

This is fixed now.