Experiment with allowing absolute paths for imports in core

[mjbvz]

For #236644

Quickly hacked this together to allow #/vs/ absolute imports. This only works for dev right now

@bpasero @jrieken Let me know if this is something we would like to look into further

[bpasero]

@mjbvz hm interesting, but:

• how does this work for auto imports that TS generates, they would be relative, no?
• not a very big fan of a leading # in imports, I prefer the relative imports in this case
• we also need to support VS Code for web, where we have a CSS dev service as well