Add eslint and stricter TypeScript rules for jspsych package

[joshunrau]

I am planning to add the option to use camel case variables to the jspsych (see issue #3386). To make sure this doesn't break any existing functionality, I think it would be a good idea to enable TypeScript strict mode (I also see this is something you want on a "lucky day"). Well, it is a lucky day!

I propose to do this in stages, each adding one of the strict rules (until finally strictNullChecks, which is the really difficult one here). I also added eslint (only for the jspsych package) to help with some additional best practices checking.

This PR makes changes that bring the existing code into compliance with the standard TypeScript linting rules and noUnusedLocales TypeScript rules.

I also removed the AudioContext hack for Safari, as it looks like AudioContext has been supported in Safari for a number of years now. If this is not okay, let me know I can add it back.

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Latest commit: 9973190

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