Introduce a new unit test for WebCanvas within ThorVG.Web CI

[hermet]

ThorVG currently lacks dedicated unit tests for the WebCanvas backend in the WebAssembly (ThorVG.Web) environment. To ensure reliability and consistent behavior across platforms, we should implement and integrate unit tests specifically targeting WebCanvas APIs.

Goals

• Add a new unit test suite focused on WebCanvas APIs with WebGPU
• Integrate WebCanvas tests into ThorVG.Web's CI workflow (e.g., GitHub Actions)
• Validate rendering output correctness or execution consistency