feat: stream time statistics

[amishshah]

Please describe the changes this PR makes and why it should be merged:

Resolves #71

Adds playbackDuration to both AudioPlayer and AudioResource:

• AudioResource.playbackDuration is incremented by 20ms every time an Opus packet is read from its underlying Opus stream. This value can be thought of as the progress through an audio track. It starts off at 0.

• AudioPlayer.state.playbackDuration is available in the Playing, Paused, and AutoPaused states. It is incremented by 20ms every time an Opus packet (whether from the audio resource, or a filler silence packet) is prepared for dispatch by the subscribers. This value can be thought of as the "real" playback time, including any transmission gaps. It resets back to 0 when playing a new resource.

You can therefore derive the length of time that filler silence has been played as the difference between these two quantities.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)

[codecov]

Codecov Report

Merging #75 (86e7568) into main (c4fa5a6) will increase coverage by 0.83%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   28.98%   29.81%   +0.83%     
==========================================
  Files          15       15              
  Lines         590      597       +7     
  Branches      131      132       +1     
==========================================
+ Hits          171      178       +7     
  Misses        417      417              
  Partials        2        2              

Impacted Files	Coverage Δ
src/audio/AudioPlayer.ts	77.51% <100.00%> (+0.35%)	⬆️
src/audio/AudioResource.ts	43.47% <100.00%> (+15.70%)	⬆️

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