Is your feature request related to a problem? Please describe.
Music that should have higher priorities 1. Don't, and 2. Can't because of the current priority system

Describe the solution you'd like
Either 1. Make a new priority tier for smaller structures, zones, and bosses
or 2. Make a new priority system where you put a number in front of an event, and the higher that number is the higher the priority of the event. This will allow setting custom priority for anything, and will clean up the biome tag events as well as other events that are split into separate priority categories. An example of this being done is Actual Music 5 for 1.12.2.

Describe alternatives you've considered
I can't fix this issue, boss music simply will not work in other dimensions because of the fact that the dimension music holds the same priority and overrides it.

Additional context
Ender dragon fight theme doesn't work if the End has a theme set as well as the End City theme not working under the same circumstance, nothing else Nether-related works either if the Nether has a set song (for instance, the Nether Fortress can't have a theme if you give the dimension a general theme). You have to use the dimension event instead of the biome events for dimensions because otherwise songs like rain, night, underground, etc will override them and you'll basically just be in the overworld with a different daytime theme.