LibJS: Move {Import,Export}Entry out of {Import,Export}Statement

By making these be standalone instead of nested structs, we can forward
declare them. This will allow us to stop including AST.h in some places.

Userland/Libraries/LibJS/Forward.h

@@ -163,6 +163,8 @@ class Environment;
 class Error;
 class ErrorType;
 struct ExecutionContext;
+struct ExportEntry;
+class ExportStatement;
 class Expression;
 class ForStatement;
 class FunctionEnvironment;
@@ -172,6 +174,8 @@ class GlobalObject;
 class HandleImpl;
 class Heap;
 class HeapBlock;
+struct ImportEntry;
+class ImportStatement;
 class Interpreter;
 class Intrinsics;
 class Module;