play/pause when Enter is pressed on non-dragging position slider

[shoffing]

(new PR created because #5685 was wrong branch)

Hey there, first time contributor here! Open to any and all feedback.

Changes

Background: This fixes a minor annoyance while using Jellyfin on my LG TV that does not have a play/pause media button on the remote (only an "OK" button between the navigation arrows):

Current behavior

Pressing "Enter" on the now playing position slider when it is not being dragged does nothing. This means resuming playback after scrubbing the position slider requires 4 inputs every time to manually click the OSD Play button: Down -> Right -> Right -> OK

New behavior

Pressing "Enter" on the position slider when it is not being dragged will play/pause the current media. Now resuming playback after scrubbing only requires one remote input: OK

To achieve this, I play/pause the video in a new "keydown" listener attached to the position slider in the video controller. It does not play/pause if default was prevented (which happens when the slider is dragged (left/right arrows) and Enter is pressed).

Manual testing

I tested these changes manually using LG's simulator.