Implement a basic date builtin

[AtkinsSJ]

A couple of the format sequences aren't implemented yet, because the values aren't exposed by JS, and any solution I come up with is likely to be incorrect when crossing daylight-savings boundaries. Basically, I can't wait for the Temporal API to be generally available!

[KernelDeimos]

Your mention of timezones makes me think of Tom Scott's video on the subject. It's good to have this command in here, I often use it at the bottom of scripts so if I check back later I know when they finished.

[AtkinsSJ]

I started working on some tests for this earlier, I'll submit them as a separate PR. I thought it would be a bit troublesome but then I remembered there's a mocking library for literally everything. 😆