Add color for tree.indentGuidesStroke

VS Code 1.36.0 (1) June 2019 introduced support for indent guide lines
in the tree view (2). To adapt to Nord's style and ensure it is still
distinguishable from the background when hovering with the mouse, the
new brightened comment color based on ` nord3` (GH-118) is used.

References:
  (1) https://code.visualstudio.com/updates/v1_36
  (2) https://code.visualstudio.com/updates/v1_36#_tree-indent-guides

GH-145

themes/nord.json

@@ -233,6 +233,7 @@
     "titleBar.border": "#2e344000",
     "titleBar.inactiveBackground": "#2e3440",
     "titleBar.inactiveForeground": "#d8dee966",
+    "tree.indentGuidesStroke": "#616e88",
     "walkThrough.embeddedEditorBackground": "#2e3440",
     "welcomePage.buttonBackground": "#434c5e",
     "welcomePage.buttonHoverBackground": "#4c566a",