Add a type.is method to attest that will statically assert bidirectional equality

[ssalbdivad]

Inspired by a suggestion from @colinhacks implemented in typejest.

Getting the types in place would be straightforward using the exactEquals util from @arktype/util.

The more important part will be adding logic to the type-checking phase to store data from an extra generic arg for is like:

// ok
attest("foo").type.is<string>();

// this needs to throw at runtime and have a type error,
// likely by requiring an extra parameter on is with an ErrorMessage
attest("foo").type.is<any>();