This PR adds the `PackageDescription.Basic` module, available to
Package.juvix files.
```
module Package;
import PackageDescription.Basic open;
package : Package := basicPackage;
```
The `PackageDescription.Basic` module provides a Package type that is
translated to a Juvix Package with all default arguments. It is not
possible to customize a basic package.
A basic package does not depend on the standard library, so loads much
more quickly.
Additionally this PR:
* Adds `juvix init --basic/-b` option to generate a basic Package.juvix.
* Migrates Package.juvix files that only use default arguments, or only
customise the name field, to basic Package files.
* Closes https://github.com/anoma/juvix/issues/2508
