LibJS: De-duplicate ClassFieldDefinition Records

This was defined twice, despite being the very same thing:
- ClassElement::ClassFieldDefinition
- ECMAScriptFunctionObject::InstanceField

Move the former to a new header and use it everywhere. Also update the
define_field() AO to take a single field instead of separate name and
initializer arguments.

Userland/Libraries/LibJS/Forward.h

@@ -141,6 +141,7 @@ class BoundFunction;
 class Cell;
 class CellAllocator;
 class ClassExpression;
+struct ClassFieldDefinition;
 class Completion;
 class Console;
 class DeclarativeEnvironment;