Allow referencing existing LabIcons

[bollwyvl]

elevator pitch

Provide a way to reference and inspect, but probably not change, LabIcons defined by other extensions and the client itself.

motivation

The icon_class approach is starting to not work as well as more things expect a LabIcon.

design ides

• python
  • Add an IconManager
    • probably lazily instantiated off Shell.icons
  • Allow creating a wrapper around an existing named LabIcon by name which can be used elsewhere
  • On a best-effort basis, offer:
    • list_icons
    • describe(id)
      • see Expand command lifecycle with optional arg validation, results #162
• typescript
  • for listing
    • use LabIcon's private _instances member, which keeps an id-mapped reference to every previously instantiated icon.
• docs
  • show a little autocompleting gallery of icons