Handle constants differently to fix build

[davepagurek]

The build was failing because DEGREES and degrees were ending up with the same slug, and also on the Netlify beta build where are URLs are lowercase, and both would map to beta.p5js.org/reference/p5/degrees.

To deal with that:

• I fixed the issue that was putting both into the same source file: entries in the Constants should be stored in content/reference/en/constants, but some constants are now not in the Constants module in the p5 source code. I updated the parser to automatically put stuff in the Constants module if the name is all caps.
• To deal with the clashing URLs, I've moved constants to /reference/p5/constants/[id]. So the two degrees will now have different URLs!
• The Foundation section was also missing because of Add fallback for property tags that don't fully parse p5.js#8257, so this commits the missing files.

This does not yet fix the OP API errors so the build won't immediately start working just yet.