Support strands shader variable names including double underscores

[eupthere]

Resolves #8875
Resolves #8794

Changes:

• Explicitly initialize strandsContext and add JSDoc comments for fields overview.
• Support strands variable name including double underscores for uniform / varying / shared.
• Introduce internal strands variable namespace _p5_strands_*.
• Store shaderNameMap, shaderNameState in strandsContext and shader hooks.
• Restore internal shader names like _p5_strands_0 back to the original user-facing names like __val in WebGL shader compile errors, to make debugging easier.
• Add unit test for changed code

Tests Passed:

• npm test -- test/unit/webgl/p5.Shader.js
• npm test -- test/unit/webgpu/p5.Shader.js

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated