Improve listchars and rendered whitespace

[bkircher]

Make NonText blue and Whitespace foreground gray so that it looks a bit more like in VS Code Dark+ color theme.

Closes https://github.com/LunarVim/darkplus.nvim/issues/1

Example:

[gepbird]

What about using dark_gray for both Whitespace and NonText?

[gepbird]

In my opinion this change is unnecessary, since it's been fixed in #14 (where it resembles more to vscode).