Color tokens for input fields (#210)

VS Code 1.46 [1] (May 2020) introduced the new
`inputOption.activeForeground` theme token for the foreground color of
active inputs [2] which are used in the search view and find widget.
The key has been added using `nord6` to make it more conform with Nord's
style.

[1]: https://code.visualstudio.com/updates/v1_46
[2]: https://code.visualstudio.com/updates/v1_46#_new-input-theme-colors

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

Closes GH-200

themes/nord-color-theme.json

@@ -118,6 +118,7 @@
     "input.border": "#3b4252",
     "inputOption.activeBackground": "#5e81ac",
     "inputOption.activeBorder": "#5e81ac",
+    "inputOption.activeForeground": "#eceff4",
     "inputValidation.errorBackground": "#bf616a",
     "inputValidation.errorBorder": "#bf616a",
     "inputValidation.infoBackground": "#81a1c1",