Clean up as many of the icons we ship as possible and use ThemeIcons

[bwateratmsft]

VSCode has a huge array of icons available in their codicons offering. Much of what they have overlaps with icons we have. We already use their ThemeIcons extensively, but we should try removing as many of our icons as possible in favor of ThemeIcons.

[bwateratmsft]

This might end up being a forcing function for doing this work...

[bwateratmsft]

This is now released in Docker extension version 1.10.0.