Add command to toggle between light/dark color themes (#151554)

* Added quick input action to toggle between preferred color themes

* Removed quickpick in favor of command to toggle between ligh/dark and high-contrast counterparts

* Moved toggle logic to themes.contribution

* togglePreferredTheme -> toggleLightDarkThemes

Co-authored-by: Leonardo Montini <[email protected]>
Co-authored-by: Martin Aeschlimann <[email protected]>