Modify nsIPrompt and the commonDialog code to allow destructive buttons

In #44030 (closed) we want to add a destructive styling to one of the prompt buttons.

Moreover, the new identity dialog also needs a destructive styling for the button.

Rather than create a custom xhtml for these cases, we should just modify the nsIPrompt and commonDialog code to enable the features we want.

Some of this could be upstreamed, if mozilla has a used for it: bugzilla bug 1979020.

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