Fix escaped characters in inline examples + instance mode examples

[davepagurek]

While working on processing/p5.js#7961, I noticed the following issues:

• Interleaved reference examples were coming in with escaped html, e.g. () => { ... } as () => { ... }, and weren't working
• Examples using instance mode weren't working because they were being executed before p5 gets injected into the example

This addresses those two issues:

• Example code is run through _.unescape first
• Instance mode is detected via the existence of new p5 in the code, and if so, disables p5 code injection, and goes back to the slightly-less-efficient but more predictable mode that just uses <script> tags