Add lNodeTypeToSelection function to scl-lib

[JakobVogelsang]

As a user of the scl-lib function that converts a LNodeType element and its tree structure into the object Selection. This function comes handy when using it in combination with the oscd-tree-grid component existing in the OpenSCD eco.

Requirement:

• export function lNodeTypeToSelection(lNodeType:Element): TreeSelection
• with TreeSelection being { [name: string]: TreeSelection;}