[web] Unify ui.Shader between CanvasKit and Skwasm (#191232)

Refactors `ui.Shader` (including `Gradient` and `ImageShader`) to share
core business logic between CanvasKit and Skwasm through the
`EngineShader` layer. Backend-specific implementations are intentionally
kept minimal, with both backends now only implementing `BackendShader`
to provide the final native calls.

Part of #175630.

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