AK: Make JSON parser return ErrorOr<JsonValue> (instead of Optional)

Also add slightly richer parse errors now that we can include a string
literal with returned errors.

This will allow us to use TRY() when working with JSON data.

AK/JsonValue.h

@@ -30,7 +30,7 @@ public:
         Object,
     };
 
-    static Optional<JsonValue> from_string(StringView);
+    static ErrorOr<JsonValue> from_string(StringView);
 
     explicit JsonValue(Type = Type::Null);
     ~JsonValue() { clear(); }