Deprecate GM.setClipboard API

[ACTCD]

Since document.execCommand() has been deprecated and has unstable behavior in iOS Safari (#285).

the clipboard can only be accessed if the user then explicitly interacts with the platform UI (ref)

We believe that in most cases, accessing the clipboard should be included in the user interaction behavior.

So we encourage using the new Clipboard API directly, and the GM.setClipboard API will no longer needed.

We plan to deprecate it in the future unless we receive use cases sufficient to consider extending support. Maybe add a deprecation prompt in v5.0 and then remove in the v6.0.