Allow extensions to use theme colors

[Tyriar]

Some extensions contribute ui for example in html preview and decorations. It would be good if those colors could be sourced from the theme. Imagine you have a test coverage extension that out green/red indicators next to the line. These colors could be based on the diff editor addition/deletion colors.

[aeschli]

I added new API to vscode.d.ts: colors in decoration options. Check out the new type ThemeColor. It stands for a reference to a color in the color registry and can be used in decorations.
https://code.visualstudio.com/docs/getstarted/theme-color-reference has the list of known colors.
Using a theme color is preferred over a custom color as it gives theme authors and users the possibility to change the color.