Change default light and dark themes to the v2 themes

[aeschli]

The new experimental themes will become the new default themes

• find a new name for the new themes
• show a notification the first time an existing user sees the new theme

[sean-mcmanus]

@aeschli With today's Insiders (on Windows), the notification shows VS Code now ships with a new default theme ''. for me, i.e. it's missing the theme name (i.e. Dark+ Experimental).