Adds a button and menu to manipulate playback speed on Spotify Web.

Quickly change playback speed on Spotify Web because some songs just sound better faster or slower.

-- Release notes --

December 5, 2023:
* Fixed preserve pitch setting not being remembered
* Fixed playback rate not being set right away when changing value

August 17 & 20, 2023:
* Fixed a bug from a recent Spotify Web update that caused the extension to not show up.

August 15, 2023:
* Added more checks to ensure a bug causing playback speed of 0 doesn't get set.

August 3, 2023:
* Fixed a bug that was causing potential audio artifacts due to unintended playback speed changes coming from a recent Spotify Web update.
  - Big thanks to Kamil K. for the help!"