add p5.js to allow friendly error messages

[schellenberg]

For beginning programmers, having p5js use the Friendly Error System (FES) by default is very useful for basic debugging. A description of the FES can be found at https://github.com/processing/p5.js/blob/main/contributor_docs/friendly_error_system.md

These changes add the un-minified version of p5.js to the project template, adds it to the list of template files in extension.ts, links to it by default in the index.html file (with comments explaining why/how to use p5.js or p5.min.js), and adds a p5.js download to the shell script that updates the p5 libraries.