Allow pre-configured customization of themes

[golf1052]

When testing editorError.background, editorWarning.background, and editorInfo.background I decided that I didn't want to include them in the theme because I found they added too much clutter. Some users may want these theme colors but it's currently impossible for the extension to include these because the themes are pre-generated. Users may manually update their themes using the workbench.colorCustomizations setting but it would be way more user friendly for our extension to handle it as users would not need to match their custom colors with the theme they're using and they wouldn't have to update their custom colors if they changed themes.

Adding support for this would require some way of generating themes when the user requests them and also denoting somewhere what color customizations can be included into a custom theme pack.