Use new static for submenu

[lachlankrautz]

On add submenu use "new static" not "new self"

This way a subclass of CliMenuBuilder will add sub menus of the same subclass type and not the parent class type.

This makes it easier to meaningfully extend.

[codecov-io]

Codecov Report

Merging #68 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #68      +/-   ##
============================================
+ Coverage     88.76%   88.79%   +0.03%     
- Complexity      239      241       +2     
============================================
  Files            18       18              
  Lines           730      732       +2     
============================================
+ Hits            648      650       +2     
  Misses           82       82

Impacted Files	Coverage Δ	Complexity Δ
src/CliMenuBuilder.php	98.7% <100%> (ø)	45 <0> (+1)	⬆️
src/CliMenu.php	89.18% <0%> (+0.07%)	55% <0%> (+1%)	⬆️

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[AydinHassan]

Thanks!