Update index.d.ts

[lifeiscontent]

This PR fixes a few things.

1. forces the T type to conform to have whatever property is set within the config
2. adds access to node.children
3. adds access to node.model
4. adds access to node.parent
5. allows extension of the config type from the TreeModel "class" so you don't need to add unknown property index

[Natan-Cruz]

Hey, no chance of maintainer merging this? I'm thinking about making a fork with your changes