Color tokens for keybinding labels (#217)

VS Code 1.56 [1] (April 2021) introduced new color tokens for keybinding
labels [2].
These have been added to make them conform with Nord's theme style:

- `keybindingLabel.background` - keybinding label background color used
  to represent a keyboard shortcut.
- `keybindingLabel.foreground` - keybinding label foreground color used
  to represent a keyboard shortcut.
- `keybindingLabel.border` - keybinding label border color used to
  represent a keyboard shortcut.
- `keybindingLabel.bottomBorder` - keybinding label border bottom color
  used to represent a keyboard shortcut.

[1]: https://code.visualstudio.com/updates/v1_56
[2]: https://code.visualstudio.com/updates/v1_56#_keybinding-label-colors

Co-authored-by: Sven Greb <[email protected]>

Closes GH-208

themes/nord-color-theme.json

@@ -140,6 +140,10 @@
     "inputValidation.infoBorder": "#81a1c1",
     "inputValidation.warningBackground": "#d08770",
     "inputValidation.warningBorder": "#d08770",
+    "keybindingLabel.background": "#4c566a",
+    "keybindingLabel.border": "#4c566a",
+    "keybindingLabel.bottomBorder": "#4c566a",
+    "keybindingLabel.foreground": "#d8dee9",
     "list.activeSelectionBackground": "#88c0d0",
     "list.activeSelectionForeground": "#2e3440",
     "list.inactiveSelectionBackground": "#434c5e",