[emscripten] Convert filter classes to C++

[trev]

Background

See #4122 for the rationale, with Emscripten we should get a significantly more compact representation of the filters data.

What to change

Reimplement most of the current API in C++. Quite a few properties don't need to be exposed however, these are only used internally anyway. Some other properties are read-only now even though these technically allowed write access before. Other differences:

• JavaScript code obtaining references to filters has to call filter.delete() in order to release the reference.
• No support for legacy "simplified" element hiding syntax, it is deprecated and should be removed now.
• No Filter.subscriptions property at this point (to be added once subscription classes are implemented).
• Filter.serialize() returns a string instead of adding to a buffer (to become an internal method anyway once FilterStorage has been converted).
• Filter.normalize() no longer exists, normalization is a mandatory part of the filter creation process.
• Filter.fromObject() in unimplemented (to become an internal method once FilterStorage has been converted).
• Filter.knownFilters lookup table replaced by static Filter.getKnownFilter() method.

[abpbot]

A commit referencing this issue has landed:
​Issue 4125 - [emscripten Convert filter classes to C++]