Add support for flutter shaders

[YaroCabbage]

What does this change?

Flutter has had shader support for quite some time now. However, this package does not implement working with them. This PR aims to solve this problem.

I had to make this addition quickly, perhaps it is too rough, but still there was an attempt to do the maximum. I apologize for not creating an issue.

I added support even for what Flutter itself most likely has not implemented yet - flavor for shaders, but nevertheless. According to the settings, I implemented an almost complete copy of regular assets. It would be possible to simply pass shaders to the asset generator, but I decided to make a copy - perhaps this is excessive.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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 test)
  • Ensure the analyzer and formatter pass (melos run format to automatically apply formatting)
• Appropriate docs were updated (if necessary)

[AlexV525]

Just a quick look on my phone without reviewing generators implementation.

[YaroCabbage]

Just a quick look on my phone without reviewing generators implementation.

Fixed @AlexV525