Fix `currentFileInfo` and `index` properties on nodes (#3602)
Fix `currentFileInfo` and `index` properties on nodes (#3602)

These fields are accessed by other tools such as parcel (for resolving local
asset URLs). As per 257efdd6884268af27beaceda467fd3a4aea5e08 the behavior with
(at least) parcel for relative paths in sub-directries changed. Prior to that
commit (last release 3.12.2) assets were resolved relative to the less module
that contains the `url(..)`. From release 3.13.0 parcel resolves assets relative
to the root less module, because no `currentFileInfo` is available.

This is caused by tree nodes setting their prototype to an instance of
`Node`. This leaves the `self` reference in `Node`s constructor pointing to the
prototype, not the actual instance the data is set on. Replacing this with
properties defined on `Node`s prototype fixes this.
less/less.js,less/less.js,less/less.js