chore: unify the handling of list variable shadows

[triceo]

All shadow variables related to a list variable are now handled in one unified loop, as opposed to each having its own variable listener.
This should technically be a performance improvement, but in fact a barely significant regression of 3 % was measured in some cases. (Other cases show minor improvements.)

This change also makes internalized previous/next shadows possible, and that in turn will help with the revamp of shadow variables that is currently in development.
Therefore we introduce this change even though it may bring a small performance regression.

[triceo]

@Christopher-Chianelli Since you're going to be using this later, would you mind reviewing?

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
78.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Christopher-Chianelli]

LGTM; possible performance improvements:

• Merge NextElementVariableProcessor and PrevElementVariableProcessor into a single, final class, and add a private final int field that +1 for NextElementProcessor, and -1 for PrevElementProcessor.
• Do not call getInverseSingleton and getIndex in getLocationInList, since they both do a map lookup on the same element.