ExprBoundingBoxesOfVoxelShape needs to override isSingle

[sovdeeth]

Describe the bug
Using ExprBoundingBoxesOfVoxelShape on a single voxel shape of more than 1 bounding box returns multiple boxes, but isSingle was true so !set {a} to bounding boxes of voxel shape of target block passes without issue, but {a} is only 1 of the bounding boxes.

To Reproduce
!set {a} to bounding boxes of voxel shape of target block while looking at stairs

Expected behavior
A parse error, since you can't set a single var to multiple things.

Screenshots
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/sk info:

[01:24:30 INFO]: [Skript] Skript's aliases can be found here: https://github.com/SkriptLang/skript-aliases
[01:24:30 INFO]: [Skript] Skript's documentation can be found here: https://docs.skriptlang.org/
[01:24:30 INFO]: [Skript] Skript's tutorials can be found here: https://docs.skriptlang.org/tutorials
[01:24:30 INFO]: [Skript] Server Version: 1.21.10-DEV-080a72f (MC: 1.21.10)
[01:24:30 INFO]: [Skript] Skript Version: 2.13.1 (skriptlang-github)
[01:24:30 INFO]: [Skript] Installed Skript Addons:
[01:24:30 INFO]: [Skript]  - skript-reflect v2.6.1 (https://github.com/SkriptLang/skript-reflect)
[01:24:30 INFO]: [Skript]  - Lusk v1.3.11 (https://github.com/JakeGBLP/Lusk)
[01:24:30 INFO]: [Skript]  - SkBee v3.13.3 (https://github.com/ShaneBeee/SkBee)
[01:24:30 INFO]: [Skript]  - oopsk v1.0-beta2 (https://github.com/sovdeeth/oopsk)
[01:24:30 INFO]: [Skript] Installed dependencies: None

Additional context
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