Support for special / static builtin requires

[Filip Tibell]

In Lune we use a special require syntax for builtin libraries instead of globals to avoid polluting the global environment and taking up useful names. Our requires are in the format require("@lune/builtin-library-name") and are statically resolvable to a table of functions/and or properties. It would be great to have support for this in selene!

[Wez Furlong]

In wezterm and KumoMTA the builtin (implemented in internal rust code) environment-specific libraries are imported via require 'wezterm' or require 'kumo' and I'd like to be able to define selene libraries for these things.