Prepare for standardization of Temporal

[eyelidlessness]

ODK XForms specifies a variety of date/time/datetime XPath functionality, much of which we have implemented with a polyfill for the Temporal proposal.

We haven’t since updated that polyfill (my bad! I should have tracked this much sooner). While Temporal is not yet supported by any of our target browsers/runtimes, it is Stage 3:

The proposal has been recommended for implementation. No changes to the proposal are expected, but some necessary changes may still occur due to web incompatibilities or feedback from production-grade implementations.

To do:

• Update to one of the polyfills recommended for production, making updates to our usage as necessary
• Establish a plan for phasing out the polyfill as runtimes implement pertinent aspects of the proposal
• Periodically check for updates to both of the above until they become moot (at which point we can close this issue)

I don’t expect usage updates to be a terribly heavy lift, and there should be little guesswork involved as I believe we have excellent test coverage of the affected functionality.