Deprecated color token for focused background of list/tree UI elements (

#215)

VS Code 1.54 [1] (February 2021) introduced a breaking change for the
focused background color of list/tree UI elements by deprecating the
`list.focusBackground` in favor of the `quickInputList.focusBackground`
theme color token [2]. The advised adaption for theme authors is to
simply adopt the same value of the deprecated token.

[1]: https://code.visualstudio.com/updates/v1_54
[2]: https://code.visualstudio.com/updates/v1_54#_updated-listtree-ui

Co-authored-by: Sven Greb <development@svengreb.de>

Closes GH-206

themes/nord-color-theme.json

@@ -211,6 +211,7 @@
     "pickerGroup.border": "#3b4252",
     "pickerGroup.foreground": "#88c0d0",
     "progressBar.background": "#88c0d0",
+    "quickInputList.focusBackground": "#88c0d099",
     "quickInputList.focusForeground": "#2e3440",
     "sash.hoverBorder": "#88c0d0",
     "scrollbar.shadow": "#00000066",