Fix p5.strands uniform calls, add instance mode construct

[davepagurek]

Resolves #7960

Changes

• Allows uniformVector2(() => [a, b]) syntax to work again
• Added support for instance mode in order to create tests
  • This involves adding the API shader.modify(() => { .... }, { a, b }) to let variables a and b work in the callback. To do this though, I've removed the options object that turns off rewriting. @lukeplowden do you think that's ok?
• Updates shader.modify() docs:
  • they now mention p5.strands as the primary API
  • they mention how to pass in local variables, e.g. for instance mode
  • pure GLSL hooks are now shown only at the end
  • examples are interleaved with the reference now so that you see the p5.strands-only examples first, and the advance cases later
  • I'll maybe update the examples a bit more before merging; it's quite simple currently

Live: https://editor.p5js.org/davepagurek/sketches/3U6iPl83z

PR Checklist

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