Render whitespace characters only for selected text

Updated to the new `selection` value for the `editor.renderWhitespace`
field introduced in VS Code 1.37 (1) that allows to only render
whitespace characters on selected text.

References:
  (1) https://code.visualstudio.com/updates/v1_37#_show-whitespace-in-selection

Resolves GH-213

snowblocks/visual-studio-code/settings.json

@@ -22,7 +22,7 @@
   "editor.lightbulb.enabled": false,
   "editor.multiCursorModifier": "ctrlCmd",
   "editor.renderIndentGuides": false,
-  "editor.renderWhitespace": "all",
+  "editor.renderWhitespace": "selection",
   "editor.smoothScrolling": true,
   "editor.snippetSuggestions": "top",
   "editor.tabCompletion": "onlySnippets",