Add barrierFilter to CP Style to set the filter being used by the Mod…

[TNorbury]

closes #25

Adds a barrierFilter to CommandPaletteStyle to set the ImageFilter that will go behind the command palette when it's open.

Luckily ModalRoute, which is what wraps around the command palette when it's open, has an argument filter in its constructor, so this ended up being a super simple change 🥂

[codecov-commenter]

Codecov Report

Merging #27 (007defe) into development (b3e5e7f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development      #27      +/-   ##
===============================================
+ Coverage        96.82%   96.84%   +0.01%     
===============================================
  Files                9        9              
  Lines              504      507       +3     
===============================================
+ Hits               488      491       +3     
  Misses              16       16              

Impacted Files	Coverage Δ
lib/src/command_palette.dart	93.38% <100.00%> (+0.11%)	⬆️
lib/src/widgets/command_palette_modal.dart	97.18% <100.00%> (+0.04%)	⬆️

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