build: replace null safety dart style package

[lcdsmao]

What does this change?

dart_style has published null-safety version.
Use pub version instead of copying source code.

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

CI Pass

[codecov]

Codecov Report

Merging #110 (c680c5f) into main (63409c3) will increase coverage by 45.66%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #110       +/-   ##
===========================================
+ Coverage   52.19%   97.86%   +45.66%     
===========================================
  Files          43       18       -25     
  Lines        3728      563     -3165     
===========================================
- Hits         1946      551     -1395     
+ Misses       1782       12     -1770     

Impacted Files	Coverage Δ
packages/core/lib/flutter_generator.dart	93.54% <ø> (ø)
packages/core/lib/generators/assets_generator.dart	100.00% <ø> (ø)
packages/core/lib/generators/colors_generator.dart	98.21% <ø> (ø)
packages/core/lib/generators/fonts_generator.dart	100.00% <ø> (ø)

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[wasabeef]