Add secondary button styles

[miguelsolorio]

VS Code will start using the secondary button style in custom dialogs. These are visible on Windows and on the web (i.e. Codespaces). To see this:

1. Install Insiders 1.56
2. Set "window.dialogStyle": "custom", in your user settings
3. Try to close an unsaved file that will prompt the dialog below

Since this is a newer color token, you'll need to add a few tokens:

• button.secondaryBackground
• button.secondaryForeground
• button.secondaryHoverBackground