Add Period Start and End functions to Date and DateTime

[radeusgd]

Pull Request Description

Implements https://www.pivotaltracker.com/story/show/183081152

Important Notes

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the
  Scala,
  Java,
  and
  Rust
  style guides.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH
    ./run ide build and ./run ide watch.

[radeusgd]

@jdunkerley do you think I should add some doc comments? Seems a bit 'naked' without them but OTOH I'd say the name of the function seems to be pretty self explanatory so I'm not sure if the comments would add any clarity. What do you think?

[4e6]

Thanks for updating the SBT build formatting

[jdunkerley]

One minor rename and a suggestion.
But looks good.

[radeusgd]

I could not fully test the autumn DST switch case as the overlapped timelines are not handled properly in polyglot which makes our Enso Date_Time unable to express one of the two instants in time there (2:30 am after the DST switch is not expressible, it gets converted back to 2:30 am before switch which is an hour earlier).

Reported this at oracle/graal#4918