AK: Implement a naive JSON parser.

This parser assumes that the JSON is well-formed and will choke horribly
on invalid input.

Since we're primarily interested in parsing our own output right now, this
is less of a problem. Longer-term we're gonna need something better. :^)

AK/JsonValue.h

@@ -21,6 +21,8 @@ public:
         Object,
     };
 
+    static JsonValue from_string(const StringView&);
+
     explicit JsonValue(Type = Type::Null);
     ~JsonValue() { clear(); }