PAINTROID-793 implement watercolor tool

[Amit-Matth]

Introduced Watercolor Tool with improved stroke rendering.

PAINTROID-793

New Features and Enhancements

• Added the Watercolor Tool with soft, brush-like stroke effects using blur.
• Integrated Watercolor Tool into the toolbox and stroke options UI.
• Implemented proper undo/redo support for watercolor strokes.
• Added unit and integration tests to verify tool behavior and rendering.

Refactorings and Bug Fixes

• Improved tap/dot drawing behavior for path-based tools.
• Updated CommandManager to better identify watercolor commands.
• Ensured accurate rendering and caching of paths while drawing.

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[bakicelebi]

Saving a water colored projects has bugs. It shows the drawn lines as full opaque brush lines.

[bakicelebi]

I also get this error on iOS simulator:

[ERROR:flutter/impeller/entity/contents/contents.cc(122)] Break on 'ImpellerValidationBreak' to inspect point of failure: Contents::SetInheritedOpacity should never be called when Contents::CanAcceptOpacity returns false.

This is probably an easy fix.

[Amit-Matth]

I fixed the bugs.