[flutter_svg] Add `SvgPicture.precompiled` constructor

[nilsreichardt]

flutter_svg allows precompiled SVG files, see Precompiling and Optimizing SVGs.

However, as a developer, you have to add three things to your code:

1. Add vector_graphics as a direct dependency
2. Add import 'package:vector_graphics/vector_graphics.dart'; import
3. Use additionally the AssetBytesLoader class.

With SvgPicture.precompiled(String assetName, [...]) a developer of the package can use precompiled SVG files without the three steps above.

Closes https://github.com/flutter/flutter/issues/177068

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[nilsreichardt]

@domesticmouse Thanks for your quick review! Do you have feedback for the questions that I posted?

[domesticmouse]

PTAL @stuartmorgan-g

[stuartmorgan-g]

PTAL @stuartmorgan-g

Since this is expanding the API surface, it should be signed off by someone from the engine team, since that's the team that has primary ownership of the package. I'll leave a comment on the issue.

[stuartmorgan-g]

From triage: @chinmaygarde Are the packages in this repo owned by the engine team being included in engine team triage? (If not, the triage-engine label is an easy way to pick up all of them with one query).

[stuartmorgan-g]

From triage: Ping @chinmaygarde on the question above.

[chinmaygarde]

We recently found that the filters used for issue triage (go/flutter-engine-triage-playbook) were ignoring SVG and vector graphics issues. I think a similar fix needs to be applied for PR triage. While the label is good, we were not looking in this repo for PRs.