Create a subcommand in `extension` for "activating" built-in extensions

[scotttrinh]

Using a new extension currently requires that a developer knows how to write the using extension [ext-name]; statement in their schema and that they need to now generate and apply a migration before it's ready to use. We should add a CLI command that is sugar for this.

Notes

• Naming ideas: use, using, install, add
• Maybe add a --no-migration switch
• Detect dev-mode and skip migration
• Add a new ext.gel file and add extensions there instead of default.gel?

[scotttrinh]

Maybe install should also imply use? Or maybe overload install to be this command which will do the standalone extension installation and "activate" it, or for built-in extensions, just activate it?