Plain Date and Time support

The [upcoming Temporal API](https://tc39.es/proposal-temporal/) being proposed at TC39 has the concept of ["Plain" date and time](https://tc39.es/proposal-temporal/docs/plaindatetime.html) representation, that is, absolute values not associated with any timezone.

This is useful on many scenarios specifically when building calendars, where there is need for iterating through days and times without considering any timezone (for building the UI around events for example).

Is there a plugin already built to solve for this kind of use case? Is this even achievable within Day.js inner state?

Start by reading the linked TC39 Temporal proposal, especially the Plain date and time representation, then inspect how Day.js represents dates internally. Determine whether the requested timezone-independent behavior fits the existing inner state or requires a plugin. Done means documenting a concrete implementation scope or confirming that an existing plugin already covers the use case.

Written by the indexing model from the issue text.