Minimap background, slider and Git state colors (#179)

Added support for the new minimap background, slider and Git state
colors [1] that were introduced in VSCode version 1.43.0 [2]
(February 2020) and version 1.41.0 [3] (November 2019).

[1]: https://code.visualstudio.com/api/references/theme-color#minimap
[2]: https://code.visualstudio.com/updates/v1_43#_minimap-background-and-slider-colors
[3]: https://code.visualstudio.com/updates/v1_41#_minimap-decorations-for-errors-warnings-and-content-changes

Resolves GH-178

themes/nord-color-theme.json

@@ -135,7 +135,17 @@
     "merge.incomingHeaderBackground": "#8fbcbb66",
     "merge.incomingContentBackground": "#8fbcbb4d",
     "merge.border": "#3b425200",
+    "minimap.background": "#2e3440",
+    "minimap.errorHighlight": "#bf616acc",
     "minimap.findMatchHighlight": "#88c0d0",
+    "minimap.selectionHighlight": "#88c0d0cc",
+    "minimap.warningHighlight": "#ebcb8bcc",
+    "minimapGutter.addedBackground": "#a3be8c",
+    "minimapGutter.deletedBackground": "#bf616a",
+    "minimapGutter.modifiedBackground": "#ebcb8b",
+    "minimapSlider.activeBackground": "#434c5eaa",
+    "minimapSlider.background": "#434c5e99",
+    "minimapSlider.hoverBackground": "#434c5eaa",
 
     /* `notification.*` keys are legacy support for VS Code versions >1.21.0 */
     "notification.background": "#3b4252",