Refactor pyodide/transcrypt template files to inherit from base ones

[berinhard]

The templates files listed under pyp5js/templates/transcrypt and pyp5js/templates/pyodide are very similar. The similar files are:

• base_sketch.py.template - after Remove from pyp5js import *  #183 mege, transcrypt version is identical to pyodide's one.
• index.html - are very similar, with minor differences. Probably a simple base.html with some blocks can do the trick here.
• target_sketch.js.template / target.py.template - this is the mos complicated one. It requires a refactoring on Transcrypt mode to have all of it functionalities within target file. That way we'll probably be able to have a base_target.py with all the duplicated code to bind p5.js under pyodide/transcrypt mode