Allow strokes to not be affecting by a scale transformation

The following description is one possible approach to implementing this, but it has not been decided if this is how it should be done. Anyone looking to implement this will first need to clear it with @Keavon from a design angle. See [this](https://github.com/GraphiteEditor/Graphite/pull/2961#issuecomment-3239632940). The description proposed by @chbornman is as follows:

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## Summary
Add a "Scaling" field to the Stroke node that allows users to toggle whether strokes should scale with shape transformations or maintain a constant visual width.

## Motivation
Currently, strokes always scale with shape transformations. This feature gives users control over stroke behavior, which is a common requirement in vector graphics applications. Users can choose between:
- **Scale with Shape**: Strokes scale proportionally with transformations (default)
- **Non-Scaling**: Strokes maintain constant visual width regardless of transformations

## Implementation Details
- Add `StrokeScaling` enum with `ScaleWithShape` and `NonScaling` options
- Add new "Scaling" parameter to the Stroke node
- Update document migration to handle the new parameter
- Fix migration bug that caused index out of bounds error when upgrading existing documents

まず、提案されている Stroke ノードの変更とリンク先の pull request の議論を確認し、実装前に @Keavon と動作をすり合わせてください。受け入れられたスケーリング設計を定義し、新しいパラメーターに対応するようドキュメントのマイグレーションを更新して、既存のドキュメントで index-out-of-bounds マイグレーションバグが発生しなくなったことを確認してください。

索引モデルが issue の本文から書いたものです。