Allow for pglite extensions, sort migrations, and add default postgres user

[salim-runsafe]

Your fork is a promising substitute for prismock, thanks for all of your work on it.

We ran into a few small things worth expanding, which this MR does:

1. Allows a user to specify a pgLiteExtensions anywhere they could already specify usePgLite and provide an array of extension names like ['citext', 'pg_trgm'] which will be imported from @electric-sql/pglite/contrib/<name>.
2. Sort migrations in the same order that prisma will apply them (alphanumerically) before applying them to pglite. When there are migrations which depend on that ordering this change will keep them deterministic.
3. Adds the default postgres user to make the default pglite DB look more like a default postgres DB.