Unset scopes from transformers after transforming to avoid memory leak

[KorvinSzanto]

This change ensures scopes don't remain set against transformers for longer than they need to be.

There were a few different options I considered:

1. Unsetting the scope after finishing transform (This is what I chose)
2. Using a \WeakReference on the transformer rather than referencing the scope directly
3. Removing the ->setCurrentScope entirely and using \Reflection(Function|Method) to determine how and when to pass the Scope to transform calls

Option	Prevents Leaks	Backwards compatible	Doesn't impact performance	Ideal DX
Unsetting	✅	✅[1]	✅	❌
`\WeakReference	✅	❌[2]	✅	❌
\Reflection	✅	❌	❌[3]	✅

• [1]: This change makes it so that you can't access the current scope after transform has completed. Instead the author of the transformer needs to manually capture the scope themselves using ->getCurrentScope() during the transform if they need to have the scope available so late.
• [2]: The only impact is that Transformer authors would no longer be able to access $this->currentScope, which isn't really a huge deal given that we already have ->getCurrentScope() available, but it is a BC break.
• [3]: While the impact is minimal, there is certainly an impact to using reflection for every single ->transform / ->include... call.

Replaces #563, #569