Activity Bar indicator styles (#175)

VS Code 1.40 [1] (October 2019) introduced a new indicator for the
active item in the Activity Bar [2] to make it stand out better and
increase readability. This includes an optional background color for
the active element.

The new theme keys `activityBar.activeBorder` and
`activityBar.activeBackground` have been added to make it conform with
Nord's theme style.

[1]: https://code.visualstudio.com/updates/v1_22
[2]: https://code.visualstudio.com/updates/v1_40#_activity-bar-indicator

Closes GH-174

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

Author: arcticicestudio

themes/nord-color-theme.json

@@ -8,6 +8,8 @@
     "activityBar.background": "#2e3440",
     "activityBar.dropBackground": "#3b4252",
     "activityBar.foreground": "#d8dee9",
+    "activityBar.activeBorder": "#88c0d0",
+    "activityBar.activeBackground": "#3b4252",
     "activityBarBadge.background": "#88c0d0",
     "activityBarBadge.foreground": "#2e3440",
     "badge.foreground": "#2e3440",