[Chapel Merge] Add datetime.timeSinceEpoch() type method

Branch: refs/heads/master
Revision: 2d134c3
Author: daviditen
Log Message:

Merge pull request #17395 from king-11/unixEpoch

Add datetime.timeSinceEpoch() type method

[PR by @king-11, reviewed and merged by @daviditen]

Add type a method to return the time since the Unix epoch.

We may choose to change this into a standalone function or a
method on a datetime object, but for now it's a type method on
the datetime type.

Modified Files:
M modules/standard/DateTime.chpl

Compare: https://github.com/chapel-lang/chapel/compare/f04c9bd4d1b9...2d134c30171e