panel: Refactor panel indicator loading for asynchronous imports

When the Shell ports to ESM, our inline imports of the network and bluetooth indicators will no longer be possible, refactor panel loading to work asynchronously.

I considered using top level await for this issue, but unfortunately the panel indicators import main.js, which imports panel.js, which creates a circular promise which can't resolve as they all require one another.

Preparation for !1499 (merged)