Add first and last day of year methods

[PacificBird]

Simple convenience methods for getting the start of the first and last days of the year for Date, PrimitiveDateTime, and OffsetDateTime. This is an easy ergonomics boost for the library, and is an infallible operation that is currently considered fallible with the existing API (using replace_ordinal).

I didn't make a separate test file for these, but since the doc-tests cover the gamut of the operation I hope that's okay. If there actually are more cases and you'd like a test suite let me know.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.8%. Comparing base (ac010e8) to head (9554c41).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
time/src/date.rs	0.0%	12 Missing ⚠️
time/src/offset_date_time.rs	0.0%	12 Missing ⚠️
time/src/primitive_date_time.rs	0.0%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #770     +/-   ##
=======================================
- Coverage   94.2%   93.8%   -0.4%     
=======================================
  Files         96      97      +1     
  Lines      10732   11172    +440     
=======================================
+ Hits       10107   10475    +368     
- Misses       625     697     +72     

☔ 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.

[jhpratt]

As stated in the "contributing" section of the README, it's best to open issues before working on an implementation. To put it simply, this is far from an ideal API because it's a method rather than an inherent function. Changing the time to midnight doesn't make any sense, either.

I would be open to something like start_of_year and end_of_year, but those would necessarily be fallible to handle validating the year range. For something infallible, the year validation would have to occur another way. I have plans to introduce new types, but that's not my top priority at the moment.