Publish package for mu import on NPM

[erikap]

This should help editors to discover imports and provide code hints.

• The NPM package is only installed to support development. During execution code of the template will always be used.
• If code hints are desired a different import statement than 'mu' will be required (e.g. @semanticworks/template)

[nvdk]

should we manage the package in this repository to keep code duplication minimal? it probably means having to move some dependencies to the mu package

[nvdk]

did an effort at extraction here: https://github.com/nvdk/mu-helpers . This one assumes node 18, so should be combined with #51