Add a loop counter

[tastymeatball]

Loop counter will be added with this pull request.

It is now possible with a number input field to set how many times a song should loop. Because the loop counter also enables looping the previous Enable Loop was renamed to Endless Loop because this is what it basically does now.
If a brstm file has a loop the loop counter will be set to the value in the field and the endless loop will be enabled otherwise the loop counter will be set to 0 and the endless loop will be disabled.

package.json: rollup-plugin-terser was updated to @rollup/plugin-terser because rollup-plugin-terser is deprecated.

[ic-scm]

I tested this, it's interesting, but if we were going to add this as a feature, I think it should be a bit more polished.

Rather than having a loop counter just to then have the playback abruptly stop at the loop point, it would make more sense to have at least a basic fade-out feature, akin to programs such as vgmstream.

It would probably also make sense to have a display of how many loops are remaining during playback, or even to adjust the displayed length of the track based on what the loop counter was set to.

While I won't merge this into the master branch of the project for now, and I'm not sure if we'd like to incorporate the feature into the player which is after all meant to be just a basic tool for quickly previewing BRSTM files, I will for now leave this request open. Also, I'm sorry that we haven't responded to this for such a long time.