New PluginTree ID generation algorithm generates identical ID's

[colin-grant-work]

Bug Description:

More details here

cc: @tsmaeder

Steps to Reproduce:

1. Set up a pair of TS files like this:

3. Do a reference search for the variable a
4. First, note that not all references are found - in this screenshot, only two console logs are shown, because some of them have the same ID, so the tree only shows one.
5. Repeat the reference search.
6. The application will freeze and eventually crash, trapped in a loop in TopDownTreeIterator because the nodes with the same ID's end up being their own siblings, so the iteration is infinite.

Additional Information

• Operating System: RHEL
• Theia Version: 1ca8131