Specify generated assets style

[lcdsmao]

What does this change?

Add support to generate flat hierarchy assets with field name style:

• camel-case
• snake-case

What is the value of this and can you measure success?

CI pass

[codecov]

Codecov Report

Merging #33 into main will increase coverage by 0.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   95.61%   96.06%   +0.45%     
==========================================
  Files          15       15              
  Lines         365      407      +42     
==========================================
+ Hits          349      391      +42     
  Misses         16       16              

Impacted Files	Coverage Δ
lib/src/generators/colors_generator.dart	95.45% <ø> (ø)
lib/src/generators/fonts_generator.dart	100.00% <ø> (ø)
lib/src/generators/assets_generator.dart	100.00% <100.00%> (ø)
lib/src/settings/flutter_gen.dart	96.77% <100.00%> (+1.31%)	⬆️
lib/src/utils/string.dart	100.00% <100.00%> (ø)

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[wasabeef]

Good Good Good.

Work well !!