Initial SplitItem + CheckableItem support

[jtreminio]

Not yet ready to merge, pending unit tests and feedback.

[codecov-io]

Codecov Report

Merging #189 into master will decrease coverage by 0.14%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #189      +/-   ##
============================================
- Coverage     93.19%   93.05%   -0.15%     
- Complexity      486      491       +5     
============================================
  Files            27       27              
  Lines          1470     1483      +13     
============================================
+ Hits           1370     1380      +10     
- Misses          100      103       +3

Impacted Files	Coverage Δ	Complexity Δ
src/MenuItem/CheckableItem.php	86.66% <ø> (ø)	19 <0> (ø)	⬇️
src/Builder/SplitItemBuilder.php	76.31% <100%> (+2.03%)	11 <1> (+1)	⬆️
src/MenuItem/SplitItem.php	97.67% <50%> (-2.33%)	45 <0> (+2)
src/Builder/CliMenuBuilder.php	84.07% <0%> (+0.4%)	67% <0%> (+2%)	⬆️

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[AydinHassan]

Looks good to me - please add some tests and documentation 👍

[AydinHassan]

Thank you @jtreminio !