Multiple Selection Management( >2 )

## Describe your suggestion
I would like to suggest adding an integer option into the #sel command, allowing users to target a specific selection rather than just all, newest, and oldest. I can halfway grok my way through the code, and part of it would probably be adding another TransformTarget and error handling around line 363 in SelCommand.java. Eg:
```INDEX(sels -> new ISelection[]{sels[index]}, "??whatgoeshere??", index);```
The part where I lose the trail is in command management and argument parsing.

## Settings
n/a

## Context
Being able to make multiple selections would be helpful, expanding one selection down, and another up for example. This is similar to the newest and oldest, but provides target selection for more than two selections. Naturally it requires the user mentally keep track of their selections.

## Final checklist
- [x] I know how to properly use check boxes
- [x] I have not used any OwO's or UwU's in this issue.