Fix usage of fn in MediaElement.connect

[davepagurek]

Resolves #8173

Changes:

• Replaces direct accesses fn and the global p5 in the MediaElement class with internal methods that return undefined
• In the addon function that adds MediaElement, patches those internal methods to have fn and p5 usage

This is a workaround to a problem that we only have because Documentation.js does not parse classes correctly when they're defined inside of an addon function. If we could do that, then we could just reference p5 and fn from the addon function parameters. However, because of our constraints, just patching in the relevant methods is maybe the best we can do.

Live: https://editor.p5js.org/davepagurek/sketches/PN2B_ebLG

PR Checklist

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