Support args for workbench.action.selectTheme

[yume-chan]

Fixes #100460

This PR adds optional args support for the workbench.action.selectTheme command, allowing users to define keybindings that directly switch to a specified theme (instead of opening the theme picker).

Such a keybinding can only be defined by directly editing keybindings.json because there is no GUI support for args.

Some examples:

[
    {
        "key": "ctrl+t ctrl+l",
        "command": "workbench.action.selectTheme",
        "args": {
            "id": "Default Light+"
        }
    },
    {
        "key": "ctrl+t ctrl+d",
        "command": "workbench.action.selectTheme",
        "args": {
            "id": "Default Dark+"
        }
    }
]

Values of the id field should match the "workbench.colorTheme" setting in settings.json. However theme's ids are not exposed anywhere in GUI, so users can only get them by switching to the desired theme, then copying from the "workbench.colorTheme" setting (or checking package.json of the theme extension).

A keybinding without args or args.id will continue opening the theme picker, without breaking existing use cases. A keybinding with invalid args.id will show an error when invoked.

---

A side effect is that now the command palette may display one of the keybindings containing args as Preferences: Color Theme command's shortcut (despite that invoking from command palette will always open the theme picker, while the displayed shortcut will directly switch to some theme). I have filed #100830 for it. But since that issue is categorized into "Backlog Candidates", I assume it's low priority and won't block this PR.