Command palette entries should have optional icons

[zadjii-msft]

• Initially spec'd in Finalize Command Palette Spec #5674
• Omitted from initial implementation in Add support for the Command Palette #6635

Do we want these icons to be a:

• path to an image?
• a font icon?

If we want to support both, the XAML binding will probably be pretty hard.

If we want to support images only. we'll probably need to ship some images for default icons.

[zadjii-msft]

It would be really cool if we could somehow support images, Segoe MDL2 Assets icons, AND emoji icons all at the same time.

That would be cool.