Minimap search results visibility (#152)

In VS Code 1.37 (1) (July 2019) the search decorations in the minimap
(code outline) have been improved so the entire line will now be
highlighted with low opacity, and the actual match shown with high
opacity.

To customize the color the new `minimap.findMatchHighlight` UI/workbench
theme key has been added to Nord.

References:
  (1) https://code.visualstudio.com/updates/v1_37#_improved-minimap-search-results-visibility

Resolves GH-150

themes/nord-color-theme.json

@@ -129,6 +129,7 @@
     "merge.incomingHeaderBackground": "#8fbcbb66",
     "merge.incomingContentBackground": "#8fbcbb4d",
     "merge.border": "#3b425200",
+    "minimap.findMatchHighlight": "#88c0d0",
 
     /* `notification.*` keys are legacy support for VS Code versions >1.21.0 */
     "notification.background": "#3b4252",