Add text color related workbench theme keys (#159)
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) https://github.com/microsoft/vscode/issues/26298
  (3) https://code.visualstudio.com/updates/v1_13

Resolves GH-154
arcticicestudio/nord-visual-studio-code,arcticicestudio/nord-visual-studio-code