Add currentTime() method

[anattolia]

Add currentTime() method that existed in previous version.

Reference: https://github.com/processing/p5.js-sound/blob/main/src/soundfile.js#L1033

[ogbabydiesal]

I would love to do this, there's a couple of things that make this non-trivial however: looking at the original p5-sound library it adds an audioworklet that keeps track of playback position - https://github.com/processing/p5.js-sound/blob/b652826d82fc8b84087e62c95d6b04ab75712e5e/src/soundfile.js#L1357

Ideally, there would be some clean Tone.js way to do this that we could reuse, however this seems unimplemented: Tonejs/Tone.js#978

it's a little out of my depth but I'd like to give it a shot. LMK if you or anyone out there lurking this repo wants to take a look.