[WebGL] shader() | setUniform() {Examples}

[hsab]

Please refer to #3723 before merging.
I had to use --no-verify with husky to bypass the strange eslint-source issue. I'm sure merging without addressing #3723 will cause a headache for other developers.

addresses #1954

• https://p5js.org/reference/#/p5/shader
• https://p5js.org/reference/#/p5.Shader/setUniform

resolves #3720
resolves #3722

shader() example: extends the loadShader() example to allow for toggling between 2 shaders on mouse click/press.
setUniform() example: alternative to the above, using the setUniform function to modify uniforms. Based on this comment by @stalgiag on PR #3722