Refactored the AssetGenImage code to be another Integration

[bramp]

What does this change?

Refactored the AssetGenImage code to be another Integration, so that it looks more similar to SVG/Lottie/Rive/etc.

This simplifies the assets_generator, and no longer forces AssetGenImage to be output even when there are no images.

This can help ensure all integration are on level footing (perhaps helping with task 1 in this list #384)

Type of change

• Refactor to cleanup the code base.

Checklist:

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
  • Ensure the tests (melos run unit:test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)

[codecov]

Codecov Report

Merging #432 (12892d7) into main (ba28410) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #432   +/-   ##
=======================================
  Coverage   98.28%   98.29%           
=======================================
  Files          20       21    +1     
  Lines         700      702    +2     
=======================================
+ Hits          688      690    +2     
  Misses         12       12           

Files	Coverage Δ
packages/core/lib/generators/assets_generator.dart	98.04% <100.00%> (-0.11%)	⬇️
...lib/generators/integrations/flare_integration.dart	100.00% <100.00%> (ø)
...lib/generators/integrations/image_integration.dart	100.00% <100.00%> (ø)
.../core/lib/generators/integrations/integration.dart	100.00% <100.00%> (ø)
...ib/generators/integrations/lottie_integration.dart	92.85% <100.00%> (-0.25%)	⬇️
.../lib/generators/integrations/rive_integration.dart	100.00% <100.00%> (ø)
...e/lib/generators/integrations/svg_integration.dart	100.00% <100.00%> (ø)
packages/core/lib/settings/asset_type.dart	95.83% <ø> (-1.05%)	⬇️

[AlexV525]

Looks cool!

[bramp]

Fixed the merge conflict, and improved the docs. Please take a look.