Represents a variable in the AST.
See also:
https://haxe.org/manual/expression-var.html
type:Null<ComplexType>
The type-hint of the variable, if available.
name:String
The name of the variable.
@:optionaloptionalisFinal:Null<Bool>
Whether or not the variable can be assigned to.
expr:Null<Expr>
The expression of the variable, if available.