Expose connection for workers

[simolus3]

This adds the exposeEndpoint() method on the web, which returns a record of values that can be sent across workers on the web. It also adds a new method that takes this sent record to open another database connection to the same source, which then shares the existing connection and mutex.

This is useful to allow a worker to connect to a database already opened by a client, such as for the sync worker in the Powersync SDK. A remaining issue is related to lifetimes: When the original connection closes and disposes the worker, the exposed endpoint will also close. sqlite3_web internally uses web locks to detect this condition, but it's not yet exposed in this package.