Re-enable checks for final access.

[EliteMasterEric]

• Improve error messages for invalid access to final variables in scripts.
• Improve handling of properties with null or never setters (Haxe never allows assignment to never, and only allows local assignment to null).
• Re-enable the code to prevent assignment to final constants from Haxe code. This prevents issues like being able to re-assign values despite them being defined as immutable constants.

[EliteMasterEric]

Oh yeah if you're wondering why it's a draft right now it's because it doesn't fucking work

[EliteMasterEric]

It works now!

I mainly wrote this to help clean up FunkinCrew/Funkin#7238, so we can rewrite the variables to recycle them without the scripts being able to edit them from the outside.

Definitely should get some review from contributors but this should prevent people from accidentally messing with constants when they aren't supposed to.

An aside, this should be good to merge as-is, but in a future PR it might be nice to add functional metadata that, when applied to a class/method/block, skips this check (in a similar manner to @:privateAccess in Haxe source).