Calculate next/prev weekday occurrences

[HubertK05]

This PR adds functionality to improve date arithmetic. It includes:

• next_occurrence for getting the closest occurrence after a given Date,
• prev_occurrence for getting the closest occurrence before a given Date,
• nth_next_occurrence for getting the nth occurrence after a given Date,
• nth_prev_occurrence for getting the nth occurrence before a given Date,
• checked versions of the methods above,
• tests for these methods,
• docs and doctests for these methods.

Addresses #593 without closing.

[codecov[bot]]

Codecov Report

Merging #594 (624cab0) into main (52e5cee) will decrease coverage by 0.0%. The diff coverage is 90.9%.

@@           Coverage Diff           @@
##            main    #594     +/-   ##
=======================================
- Coverage   95.7%   95.7%   -0.0%     
=======================================
  Files         79      79             
  Lines       8818    8884     +66     
=======================================
+ Hits        8443    8503     +60     
- Misses       375     381      +6     

Impacted Files	Coverage Δ
time/src/date.rs	98.7% <90.9%> (-1.3%)	:arrow_down:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[jhpratt]

Looks good. Thanks!