Web support for powersync_sqlcipher

[simolus3]

Since sqlite3_web: ^0.3.0, we can pass additional options from the client to the DatabaseController when opening a database. This implementation uses that in two steps:

1. In powersync_core, introduce an additional option in the worker that will make it use multipleciphers-$vfs when we've set up a VFS named $vfs for a database.
2. In powersync_sqlcipher, add a web implementation and set that option to enable the multipleciphers-based VFS.

This ensures that we're using an encryption-compatible VFS for the database. Applying encryption is then only a matter of setting the key pragma, which we can also do in the open factory.

To aid in downloading web assets for powersync_sqlcipher, I've moved the setup_web.dart into powersync_core. Botth powersync and powersync_sqlcipher can use that library to download the web workers and WASM assets. The only difference is that powersync_sqlcipher will download sqlite3mc.wasm instead of the regular one.

Closes #216

[simolus3]

Pana is failing because powersync/powersync_sqlcipher depend on changes in powersync_core, this should get resolved once we start the releases. I've prepared the changelogs with melos version and I can push the tags after a review.