Throw if shapeKey isn't set when using pglite/sync

[KyleAMathews]

From @samwillis

We've not so far as it enables syncing in to an ephemeral database without the additional overhead. But as it's probably wanted in 95% of cases maybe we should have an unkeyedShape: true or similar that allows it.
Or maybe shapeKey: null?

Tony from Discord also suggested that:

perhaps it could be based on whether the dbDir is set?

[samwillis]

dbDir isn't enough to make a judgment as to if its a persisted database or not. It's possible to pass a custom VFS as an alternative to a dbDir, and there may be alternative in-memory VFSs in future, such as a CoW cloneable VFS for unit tests.

We have a first version of this merged this morning where its required, but passing null stops it persisting state: #522