[To Do] Scope visitors added by `@plugin`

[matthew-dean]

One thing the @plugin feature (plugins added/called from Less code) added was scoped functions. However, that scoping doesn't apply to visitors. So, if a plugin adds a visitor for nodes, those will apply to all subsequently visited nodes. A more library- / component-friendly approach would be to scope any "effects" of @plugin, including visitors and functions, to the @plugin scope.