Color token for active terminal tab indicator (#227)

VS Code 1.59 [1] (July 2021) introduced a new color token for active
terminal tab indicators [2].
It has been added to make it conform with Nord's theme style:

- `terminal.tab.activeBorder` - color for the border on the side of the
  terminal tab in the panel.

[1]: https://code.visualstudio.com/updates/v1_59
[2]: https://code.visualstudio.com/updates/v1_59#_active-terminal-tab-indicator

Closes GH-226

themes/nord-color-theme.json

@@ -281,6 +281,7 @@
     "terminal.ansiBrightMagenta": "#b48ead",
     "terminal.ansiBrightCyan": "#8fbcbb",
     "terminal.ansiBrightWhite": "#eceff4",
+    "terminal.tab.activeBorder": "#88c0d0",
     "textBlockQuote.background": "#3b4252",
     "textBlockQuote.border": "#81a1c1",
     "textCodeBlock.background": "#4c566a",