Add helper function to fix JS subkey encoding

[simolus3]

When forwarding data lines to the core extension, our JS SDK would encode subkey values as JSON (before encoding the rest of the message as JSON too).

This means that ps_oplog.key entries created by the JS SDK look like foo/bar/"baz", while all other SDKs are using foo/bar/baz directly. To prepare the adoption of the Rust sync client in the JS SDK (and for consistency with our other SDKs), we have to migrate ps_oplog entries.
The migration will happen in the JS SDK since it's the only SDK affected by this. However, the logic to strip the JSON encoding is not really expressible with builtin SQL functions alone, so this adds a powersync_remote_duplicate_key_encoding that detects improper " keys to remove them.

The idea is that the JS SDK would use an entry in ps_kv to detect whether a migration has ran and, if it hasn't, run UPDATE ps_oplog SET key = powersync_remote_duplicate_key_encoding(key); when opening the database.