between supports Date/IDate with missing bounds

[grcatlin]

Closes #7281

I believe this implementation matches @aitap's suggestion, where Date/IDates are passed to the C code directly. I tried to line up the character bound parsing with the existing implementation for POSIXct.

This is my first time contributing to data.table and I'd appreciate any feedback!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (188dd7b) to head (8dfa781).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7751   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       17065    17071    +6     
=======================================
+ Hits        16902    16908    +6     
  Misses        163      163           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.