LibJS: Update bytecode generator to use local variables

- Update ECMAScriptFunctionObject::function_declaration_instantiation
  to initialize local variables
- Introduce GetLocal, SetLocal, TypeofLocal that will be used to
  operate on local variables.
- Update bytecode generator to emit instructions for local variables

Userland/Libraries/LibJS/Forward.h

@@ -184,6 +184,7 @@ class HeapBlock;
 struct ImportEntry;
 class ImportStatement;
 class Interpreter;
+class Identifier;
 class Intrinsics;
 struct IteratorRecord;
 class MetaProperty;