feat(reify): add the ability to add a hook

[ylemkimon]

This PR adds the ability to add a hook via options.hook. It adds a single hook options.hook[Symbol.for('beforeReify')](Arborist), that runs after this[_loadTrees]() but before this[_reifyPackages](options). Other hook can be added later, with backward compatability.

Sometimes in a project, there is a need to enforce some policies on dependencies used in the project or mutate the tree. However, npm or Arborist doesn't provide a way to hook into its process.

References

Related to npm/rfcs#460

Other package managers

• Yarn 1: undocumented hooks can wrap each step (Adds an undocumented run hook yarnpkg/yarn#7557)
• Yarn 2: hooks can be added via a plugin (https://yarnpkg.com/advanced/plugin-tutorial#using-hooks)
• pnpm: hooks can be added via configuration (https://pnpm.io/pnpmfile)

[fritzy]

@ylemkimon
This repo will be archived soon. arborist is now maintained as a workspace in the CLI, but still published as its own library.
I have created a new PR at npm/cli#4260, which maintains you as the commit author.