Adopt icon font in explorer

[miguelsolorio]

Related #78889

This adopts the icon font in the explorer and some editor tabs:

These icons also use icon.foreground and when configured will look like:

I've gone through and tested the various cases (multiple editor groups + hover/dirty states, etc.) and would love it if you could give it a quick pass to ensure I didn't miss any icons. Note that I haven't done the tree chevrons and other editor toolbar items (like when tab visibility is toggled off).

[bpasero]

Adding @isidorn for explorer.

[isidorn]

@misolori nice work. Especially like how we are getting rid of css.
I have tested this out and verified this works nicely out of source and by doing a full local build of vscode -> approved.