Remove patternengine-node-mustache as shipped with core

[bmuenzenmeyer]

In PL Node 3.0 https://github.com/pattern-lab/patternengine-node-mustache will not be a dependency. Rather, our getting started / edition / CLI work should help facilitate the creation of the proper setup.

We should however, consider keeping mustache as an interrim dependency so the general-header and general-footer templates from styleguide-mustache-default can be rendered into the target template of the user's choosing.