Add text color related workbench theme keys (#159)

This PR adds some workbench color keys for text related elements
that were not supported by Nord yet.
The commit in the VS Code repository (1)
(Micorosft/vscode@752acd50) added multiple new theme key
in through microsoft/vscode#26298 (2) that was
merged over 2 years ago and released for the first time in stable
version 1.13.0 (3) but were never mentioned in the release notes.

The following theme keys have now been added:

- `textBlockQuote.background` - Background color for block quotes in
  text.
- `textBlockQuote.border` - Right-side border color for block quotes in
  text.
- `textLink.activeForeground` - Foreground color for links in text when
  clicked on and on mouse hover.
- `textLink.foreground` - Foreground color for links in text.
- `textPreformat.foreground` - Foreground color for preformatted text
  segments like inline code.
- `textSeparator.foreground` - Color for text separators.

References:
  (1) https://github.com/microsoft/vscode/blame/752acd504206f00ffe4ed101c2496540d0197f56/src/vs/platform/theme/common/colorRegistry.ts#L164-L170
  (2) microsoft/vscode#26298
  (3) https://code.visualstudio.com/updates/v1_13

Resolves GH-154

Author: arcticicestudio

themes/nord-color-theme.json

@@ -230,7 +230,13 @@
     "terminal.ansiBrightMagenta": "#b48ead",
     "terminal.ansiBrightCyan": "#8fbcbb",
     "terminal.ansiBrightWhite": "#eceff4",
+    "textBlockQuote.background": "#3b4252",
+    "textBlockQuote.border": "#81a1c1",
     "textCodeBlock.background": "#4c566a",
+    "textLink.activeForeground": "#88c0d0",
+    "textLink.foreground": "#88c0d0",
+    "textPreformat.foreground": "#8fbcbb",
+    "textSeparator.foreground": "#eceff4",
     "titleBar.activeBackground": "#2e3440",
     "titleBar.activeForeground": "#d8dee9",
     "titleBar.border": "#2e344000",