Add support to the stores for managing cascading delete on parent RELTYPE

[allenporter]

Add TodoStore and EventStore support for managing PARENT relationships types on deletions to delete all children of events.

[codecov]

Codecov Report

Merging #244 (d2f2cdd) into main (1e7ba71) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   96.88%   96.90%   +0.01%     
==========================================
  Files          45       46       +1     
  Lines        2730     2807      +77     
==========================================
+ Hits         2645     2720      +75     
- Misses         85       87       +2     

Files	Coverage Δ
ical/event.py	97.56% <100.00%> (+0.02%)	⬆️
ical/journal.py	100.00% <100.00%> (ø)
ical/store.py	98.90% <100.00%> (+0.17%)	⬆️
ical/todo.py	98.64% <100.00%> (+0.03%)	⬆️
ical/types/__init__.py	100.00% <100.00%> (ø)
ical/types/cal_address.py	100.00% <ø> (ø)
ical/types/relation.py	95.55% <95.55%> (ø)

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!