Update stave package name

[mylibrar]

Is your feature request related to a problem? Please describe.
To import StaveViewer, we need to use:

from nlpviewer_backend.lib.stave_viewer import StaveViewer

The current package name is nlpviewer_backend, which is not very indicative of a Stave package.

Describe the solution you'd like
We want to change the package name to something more intuitive for users, e.g., stave_backend.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.