More code cleanup

[whikloj]

• Add phpstan as an optional check for now
• Changes coming out of running phpstan
• Stop unset()ing to indicate a default value, just set it to the default.
• Rename validate() to isValid() to better indicate it returns a boolean.
• Add deprecation notice to validate()

[codecov[bot]]

Codecov Report

Merging #38 (e89bf3f) into main (968d359) will increase coverage by 0.84%. The diff coverage is 95.65%.

@@             Coverage Diff              @@
##               main      #38      +/-   ##
============================================
+ Coverage     87.66%   88.50%   +0.84%     
+ Complexity      479      477       -2     
============================================
  Files             7        7              
  Lines          1248     1244       -4     
============================================
+ Hits           1094     1101       +7     
+ Misses          154      143      -11     

Impacted Files	Coverage Δ
src/BagUtils.php	92.70% <ø> (ø)
src/Bag.php	85.99% <95.45%> (+1.43%)	:arrow_up:
src/Commands/ValidateCommand.php	89.18% <100.00%> (ø)
src/Fetch.php	95.04% <100.00%> (ø)

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