support context action contributions for watches and variables

[weinand]

one use case: #36704

[isidorn]

Once we get more use cases I would consider this.
Code pointer https://github.com/Microsoft/vscode/blob/isidorn/vertical_panel/src/vs/platform/actions/electron-browser/menusExtensionPoint.ts#L39

If we do this, we should also update docs

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