Color tokens for secondary buttons of custom dialogs (#216)

VS Code 1.56 [1] (April 2021) introduced new colors tokens for secondary
buttons of custom dialogs [2]:

- `button.secondaryBackground`
- `button.secondaryForeground`
- `button.secondaryHoverBackground`

This commit also includes visual improvements for the current Nord theme
styles of primary buttons:

- `button.background` - `nord2` to `nord8` with a transparency of ~93%.
- `button.foreground` - `nord4` to `nord0`.
- `button.hoverBackground` - `nord3` to `nord8`.

[1]: https://code.visualstudio.com/updates/v1_56
[2]: https://code.visualstudio.com/updates/v1_56#_updated-custom-dialogs

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

Closes GH-207

themes/nord-color-theme.json

@@ -14,9 +14,12 @@
     "activityBarBadge.foreground": "#2e3440",
     "badge.foreground": "#2e3440",
     "badge.background": "#88c0d0",
-    "button.background": "#434c5e",
-    "button.foreground": "#d8dee9",
-    "button.hoverBackground": "#4c566a",
+    "button.background": "#88c0d0ee",
+    "button.foreground": "#2e3440",
+    "button.hoverBackground": "#88c0d0",
+    "button.secondaryBackground": "#434c5e",
+    "button.secondaryForeground": "#d8dee9",
+    "button.secondaryHoverBackground": "#4c566a",
     "charts.red": "#bf616a",
     "charts.blue": "#81a1c1",
     "charts.yellow": "#ebcb8b",