When the profile icon is set to null, fall back to the icon of the commandline

[zadjii-msft]

Basically, title. If you null out the icon, we'll automatically try to use the commandline as an icon (because we can now). We'll even be smart about it - cmd.exe /k echo wassup will still just use the ico of cmd.exe.

This doesn't work for ubuntu.exe (et. al), because that commandline is technically a reparse point, that doesn't actually have an icon associated with it. Closes #705

"none" becomes our sentinel value for "no icon".

This will also use the same NormalizeCommandLine we use for commandline matching for finding the full path to the exe.

[lhecker]

• [ ] The only way to hide the icon becomes "icon": " ". That's weird. but passable?

Why can't we make it so that there's a differentiation between null and "empty string"? Empty string to hide the icon seems fine-ish to me...

[zadjii-msft]

Discussion notes:

• Add a checkbox for "none" to the SUI for this setting.
• This doesn't fix #7552 but it sure does make it easier in the future

[zadjii-msft]