Prebuild v13 accidentally removed support for node-ninja and nw-gyp, possible next steps

[lovell]

In moving to take advantage of the Promise-based API provided by node-gyp v10 in #316, we inadvertently broke the use of alternative "backends" (i.e. forks of node-gyp), namely node-ninja and nw-gyp, which still rely on callbacks.

Some possible options (there will be others):

1. Include an implicit mapping for each "backend" to determine if we need to "promisify" them.
2. Add a new, explicit --backend-uses-callbacks option that does what it says on the tin.
3. Move node-ninja and nw-gyp to optional peer dependencies and assume they will always use callbacks.

Thoughts/alternatives welcome, especially if you help maintain an open source package that depends on the node-ninja or nw-gyp forks.