Update constructors.d.ts - allow optionally undefined ParsableDate

[9ParsonsB]

currently, if you have a type that is one of the ParsableDate and optionally undefined, but not specifically a Date or undefined (i.e. Date | undefined) typescript will not allow the ParsableDate constructor as it does not allow undefined:

setDate(date?: string) {
  const st = spacetime(date)
}

No overload matches this call.
The last overload gave the following error.
Argument of type 'string | undefined' is not assignable to parameter of type 'ParsableDate'.

This PR removes one of the constructors (that is already covered by the ParsableDate constructor), and allows ParsableDate to be undefined

[spencermountain]

thank you!

[spencermountain]

released in 7.4.2