New contextmenu event definition missing bypass

[pheki]

Hi!

Before 2017 the WHATWG spec used to specify that User Agents could bypass the contextmenu event to ensure that the user can always access the default context menu, see whatwg/html#6433

Since then the contextmenu event was defined in #316, but it's now missing the possibility of bypass.

This is also implemented in Gecko. It can be verified by opening #316's test page and holding shift while right clicking in Firefox.