[Feature Request] Volume Control Increase or Decrease

### Is your feature request related to a problem? Please describe.

Thank you for your work! I use this bot to provide ambient sound during rpg sessions.

I would like to be able to control the volume a bit more dynamically.

So using a macro it would be nice if we could use something like this to increase the current volume level by 10.
$volume +10

### What is your ideal solution to the problem?

Add code to detect current volume level and if +/- is used as a qualifier on the parameter increase or decrease the volume level accordingly.

### How would this feature be used?

Better dynamic control of the volume to change mood.

### Additional Info

_No response_

### Checklist

- [X] I have checked [the documentation](https://jmusicbot.com/) to make sure this feature doesn't already exist
- [X] I have searched for [similar feature requests](https://github.com/jagrosh/MusicBot/labels/enhancement)
- [X] I have confirmed that my requests is **not** on the [list of things that will not be added](https://jmusicbot.com/things-that-wont-be-added)
- [X] I am running the latest version of the bot: 

Start by locating the existing volume command, its argument parsing, and the audio volume-control entry point. Trace how the current level is read and changed, then verify that relative values such as `$volume +10` and negative adjustments produce the intended result without exceeding valid volume limits.

Written by the indexing model from the issue text.