Color tokens for status bar warning items (#230)

VS Code 1.59 [1] (July 2021) introduced new color tokens for status bar
warnings [2].
They have been added and adjusted to match Nord's theme style:

- `statusBarItem.warningBackground` - the background color used for
  warning items in the status bar that stand out from other status bar
  entries to indicate warning conditions.
- `statusBarItem.warningForeground` - the foreground color used for
  warning items in the status bar that stand out from other status bar
  entries to indicate warning conditions.

[1]: https://code.visualstudio.com/updates/v1_59
[2]: https://code.visualstudio.com/updates/v1_59#_status-bar-warning-colors


Co-authored-by: Sven Greb <development@svengreb.de>

Closes GH-228

Author: arcticicestudio

themes/nord-color-theme.json

@@ -246,6 +246,8 @@
     "statusBarItem.prominentHoverBackground": "#434c5e",
     "statusBarItem.errorBackground": "#3b4252",
     "statusBarItem.errorForeground": "#bf616a",
+    "statusBarItem.warningBackground": "#ebcb8b",
+    "statusBarItem.warningForeground": "#2e3440",
     "statusBar.border": "#3b425200",
     "tab.activeBackground": "#3b4252",
     "tab.activeForeground": "#d8dee9",