We can now use bevy-ui-navigation 0.33 with bevy 0.12, and the chirpunk
example works again.

I settled on an import syntax. It is markedly different from the draft
one. So we needed to change the AST and parser to account for those
changes.

Of course it's broken right now.

- Now templates with a pub prefix are collected in the pub_templates Ast
  struct field
- Now, when parsing, the result may either be a scene or a template
  library.
- Template libraries are mutually exclusive with scene files.