Support for dynamic extensions

[psychemedia]

See https://lantern.dev/blog/pglite-lantern for examples of a fork that supports loading postgres extensions.

• demo: https://pglite.lantern.dev/
• code: https://github.com/lanterndata/pglite-demo
• my backup: https://github.com/ouseful-backup/pglite-demo

[psychemedia]

Alternatively, allow switches in the magoc that creates the initial pglite instance and connection to install one or more extensions.

Also have a magic that lists the available extensions.