Add theme keys for new notification center UI

VS Code 1.21 (1) (February 2018) introduced new theme keys for the new
notifications UI (2).

References:

  (1) https://code.visualstudio.com/updates/v1_21
  (2) https://code.visualstudio.com/updates/v1_21#_new-notifications-ui

GH-78

Author: arcticicestudio

README.md

@@ -112,6 +112,8 @@ Fully supported [Workbench](https://code.visualstudio.com/docs/getstarted/theme-
 
 <p align="center"><strong>Notifications</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-visual-studio-code/develop/assets/scrot-feature-ui-notification.png"/></p>
 
+<p align="center"><strong>Notification Center (supported as of VS Code version 1.21 or higher)</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-visual-studio-code/develop/assets/scrcast-feature-notification-center.gif"/></p>
+
 <p align="center"><strong>Badges</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-visual-studio-code/develop/assets/scrot-feature-ui-badges.png"/></p>
 
 <p align="center"><strong>Side Bar Views (Debug)</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-visual-studio-code/develop/assets/scrot-feature-ui-side-bar-debug.png"/></p>

assets/scrcast-feature-notification-center.gif

@@ -129,17 +129,29 @@
     "merge.incomingHeaderBackground": "#8fbcbb66",
     "merge.incomingContentBackground": "#8fbcbb4d",
     "merge.border": "#3b425200",
+
+    /* `notification.*` keys are legacy support for VS Code versions >1.21.0 */
     "notification.background": "#3b4252",
-    "notification.foreground": "#d8dee9",
     "notification.buttonBackground": "#434c5e",
-    "notification.buttonHoverBackground": "#4c566a",
     "notification.buttonForeground": "#d8dee9",
+    "notification.buttonHoverBackground": "#4c566a",
+    "notification.errorBackground": "#bf616a",
+    "notification.errorForeground": "#2e3440",
+    "notification.foreground": "#d8dee9",
     "notification.infoBackground": "#88c0d0",
     "notification.infoForeground": "#2e3440",
     "notification.warningBackground": "#ebcb8b",
     "notification.warningForeground": "#2e3440",
-    "notification.errorBackground": "#bf616a",
-    "notification.errorForeground": "#2e3440",
+
+    "notificationCenter.border": "#3b425200",
+    "notificationCenterHeader.background": "#2e3440",
+    "notificationCenterHeader.foreground": "#88c0d0",
+    "notificationLink.foreground": "#88c0d0",
+    "notifications.background": "#3b4252",
+    "notifications.border": "#2e3440",
+    "notifications.foreground": "#d8dee9",
+    "notificationToast.border": "#3b425200",
+
     "panel.background": "#2e3440",
     "panel.border": "#3b4252",
     "panelTitle.activeBorder": "#88c0d000",