[emscripten] Move impl of Subscription serialization from Subscription classes

[sergz]

It's almost duplication of #5138.

Background

The Subscription classes should not be aware how they are used, in particular how they are serialized and parsed and what format is used. In addition, parsing and serializing code would share some common parts which are likely not related to Subscription classes.

What to change

• create PatternsIniSerializer class which writes headers (e.g. version=5) and can serialize Subscription classes in INI format.
• for the sake of simplicity keep Serialize member functions but don't expose them to JS and they should merely create a \n concatenated property=value strings. SerializeFilters should be migrated into PatternsIniSerializer and Subscription class should expose the non-modifiable container of Filters, e.g. const Subscription::Filters&.