WIP: Add sync implementation from core extension

[simolus3]

This adds an option to use a new sync implementation from the core extension instead of the JavaScript implementation. The core implementation is marked as experimental, and we default to the old implementation.

The implementation changes are surprisingly compact:

1. In AbstractRemote.ts, this introduces "raw" variant of the sync stream methods. Those don't parse sync lines, instead returning them as strings (HTTP streaming) or buffers (RSocket).
2. In AbstractStreamingSyncImplementation.ts, copy the existing sync implementation into a method and add a new one for the core sync implementation.

See also:

• The implementation PR for the core extensions: github.com/Implement sync client powersync-sqlite-core#70
• The equivalent PR for the Kotlin SDK: WIP: Add sync implementation from core extension powersync-kotlin#192

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