Find alternatives for document.execCommand

[threema-danilo]

From https://developer.mozilla.org/en-US/docs/Web/API/document/execCommand:

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

On the other hand, there's no clear replacement: https://stackoverflow.com/questions/61520602/is-there-a-replacement-for-document-execcommand-or-is-it-safe-to-use-document

I don't assume the execCommand API will stop working anytime soon, but we should still take a look at our options.

The https://w3c.github.io/input-events/ spec may be an alternative to some use cases (e.g. stripping formatting on paste), but not all (e.g. inserting emoji HTML snippets). And the implementation https://github.com/w3c/input-events/ seems a bit stalled. Maybe some issues could be pinged.