Feature/instruction bar

[TNorbury]

closes #14

Adds a flag, showInstructions, to CommandPaletteConfig. Setting to true will show an instruction bar like so:

When a nested item is selected, an additional instruction will be added:

This PR also includes the exporting of a new widget KeyboardKeyIcon, which is used to make the Keyboard Keys used in this package (such as for the trailing widget for action, or in the instruction bar itself). This will allow users to mimic the keyboard shortcut widget used in the default action builder.

[codecov-commenter]

Codecov Report

Merging #20 (d3dce24) into development (f883168) will increase coverage by 3.70%.
The diff coverage is 99.05%.

@@               Coverage Diff               @@
##           development      #20      +/-   ##
===============================================
+ Coverage        93.02%   96.72%   +3.70%     
===============================================
  Files                7        9       +2     
  Lines              430      489      +59     
===============================================
+ Hits               400      473      +73     
+ Misses              30       16      -14     

Impacted Files	Coverage Δ
lib/src/widgets/command_palette_modal.dart	96.61% <ø> (ø)
lib/src/widgets/options/command_palette_body.dart	97.10% <97.36%> (ø)
lib/src/command_palette.dart	93.10% <100.00%> (+11.52%)	⬆️
lib/src/widgets/command_palette_instructions.dart	100.00% <100.00%> (ø)
lib/src/widgets/keyboard_key_icon.dart	100.00% <100.00%> (ø)

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