Feature request: highlight `meta.mainProgram`, if one is set

[acid-bong]

Reasoning: this sounds useful, when the program has a lot of executables, but a user wants to just nix run it and already know which one will run without looking at the source.

One of design suggestions: make that executable bold

Another way could be adding colour (but not blue, it's not a link/anchor)

[raboof]

Or maybe we should have a nix run tab next to nix-shell that could document which executable is ran when you nix run nixpkgs#emacs?

[raboof]

In any case this sounds like a neat improvement, happy to review any PR