Document the “icon” option of extension commands

[FranklinYu]

Some extension has commands with icons, such as GitLens (warning: large JSON file). Snippet of that section:

{
    "command": "gitlens.diffWithNext",
    "title": "Compare File with Next Revision",
    "category": "GitLens",
    "icon": {
        "dark": "images/dark/icon-next-commit.svg",
        "light": "images/light/icon-next-commit.svg"
    }
}

Is this documented somewhere? Current documentation seems to only cover command, title, and category.

[octref]

Related: #2189

@mjbvz I do think we should have a reference though. This was one of the feedback in microsoft/vscode#58226 (comment) too.

[gregvanl]

https://code.visualstudio.com/api/references/contribution-points#contributes.commands does mention icons.