Update demo editor "Run" button to be deactivated until Pyodide finishes loading

[berinhard]

Currently, if the users click on the "Run" button before the demo sketch appears, they get the following error:

Uncaught TypeError: window.runSketchCode is not a function
    runCode https://berinhard.github.io/pyp5js/pyodide/:104
    <anonymous> https://berinhard.github.io/pyp5js/pyodide/:124
    jQuery 2

But when the sketch starts to run, the run button gets back to work properly. It's probable that issue #189 will make it easier to implement this feature.

[jvfe]

A simple, but not ideal, fix would be to change these lines to be:

$("#executeBtn").on("click", () => { if (window.instance) { runCode() }; });
$("#clearBtn").on("click", () => { if (window.instance) { window.instance.remove() }; });

Then the click event would only be fired once p5's window.instance object is created.

[berinhard]

@jvfe I think what you're suggesting is very reasonable. Maybe we can add an else condition to the executeBtn logic to display an alert box with a message like: "Pyodide is still loading. Please, wait a few seconds and try to run it again."