Throw an exception when attempting to register an abstract class

[takejohn]

Description

Registering an abstract class as a syntax element doesn't cause any errors;
however, parsing a code matching it will fail with an error like "Can't understand this ..."
I'd like to add a check on the constructor parameter of SyntaxElementInfo for abstract classes so an addon developer can notice what's wrong with their addon.

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Target Minecraft Versions: any
Requirements: none
Related Issues: none

[TheLimeGlass]

There are ClassInfos such as Projectile that are registered as abstract classes

[takejohn]

There are ClassInfos such as Projectile that are registered as abstract classes

I believe ClassInfos are okay to be registered as abstract class if Skript doesn't need to instantiate them.
I want to change SyntaxElements, which are necessary to be instantiated by Skript.

[sovdeeth]

Marked as feature-ready, but I believe this is fine for a patch as well.