Render ASCII-only spinner and icons on conhost.exe

[DavidS-ovm]

Fixes #388

[DavidS-ovm]

Refactored the IsConhost() logic for more linear flow.

@dylanratcliffe please give this a spin on your windows machine.

[dylanratcliffe]

The logic is working well, we need to replace the spinner too of course but the logic works

[DavidS-ovm]

Added the missing call to PlatformSpinner() in tea_taskmodel.go

[dylanratcliffe]

Logic looks good, it will of course also need to compile on Darwin, which the current _linux and _windows doesn't allow. Given that we're not doing anything at the compiler level that doesn't work cross-platform (like making certain syscalls) I'd be tempted to just compile all the logic in rather than doing it per-version, it's only a few lines of code but up to you

[DavidS-ovm]

adding theme_darwin.go is so much easier than trying to divine whether there's a /prov/version and dealing with random environment variables.