Upgrade `--load-extension` to accept entrypoints like Datasette

Imitate:
- https://github.com/simonw/datasette/pull/1789
```
# would load default entrypoint like before
datasette data.db --load-extension ext

# loads the extensions with the "sqlite3_foo_init" entrpoint
datasette data.db --load-extension ext:sqlite3_foo_init

# loads the extensions with the "sqlite3_bar_init" entrpoint
datasette data.db --load-extension ext:sqlite3_bar_init
```