Race condition between Play button loading & Preview server being ready #4401

p5.js version

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What is your operating system?

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Web browser and version

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Actual Behavior

On a slow connection, clicking Play right after opening a sketch can silently do nothing, because the preview iframe hasn't started listening yet

This happens occassionally in e2e tests, which caused tests to flake prior to this PR

Expected Behavior

Play button should maybe be disabled till preview iframe is ready OR the app retries to send the preview iframe the sketch code until it is listening

Steps to reproduce

Steps:

Snippet:

// Paste your code here :)