Temporal Polyfill

This is an implementation of a polyfill for the TC39 Termporal Proposal.

Implementation Issues:

Proposed Changes / Additions to the Proposal

Instant.prototype.format(locale = navigator.language, options = {}) & ZonedInstant.prototype.format(locale = navigator.language, options = {})

Considering the frequency of converting date/time values to customized strings, the format convenience method seems to make sense. It should follow the same signature as Intl.DateTimeFormat.prototype.format with the notable exception that locale is defaulted to navigator.language and the timeZone option is overriddent to the ZonedInstant's time zone or UTC for Instants.