Some themes render with wrong fallback color

[octref]

From #45 (comment)

For a VS Code theme, the canonical way to specify a fallback color should be to include a settings key as first property of tokenColors with no scope, for example:

https://github.com/arcticicestudio/nord-visual-studio-code/blob/36dac6cae5996995543af99c12e23587028913a5/themes/nord-color-theme.json#L266-L272

Some theme does not do that, and vscode-textmate would generate a wrong color map.
I asked vscode-textmate in this case to use colors['editor.foreground'] and colors['editor.background'] as a fallback, but was pointed to open an issue in VS Code to fix their themes. I then opened a PR but that was closed: microsoft/vscode#105418

So here we have to do our own logic: If there's no no-scope global setting with fallback colors, we have to manually insert it into the theme so vscode-textmate can tokenize with correct color.

[vinkla]

Thanks for pushing a fix for this issue @octref! Keep up the good work 👏