Add mousewheel to the "Legacy Event Types" section

[garykac]

mousewheel is deprecated, but it present on some user agents.

We should add it and include so that we have a place to add information about the event.

See #282 and pull request #330

[zcorpan]

Also see whatwg/dom#1087 which specifies the passive default event listener intervention also for mousewheel.

[zcorpan]

I think WebKit and Chromium has some logic for only firing one of wheel or mousewheel depending on which event listeners are present (only wheel if both are present). I assume this is needed for web compat since pages may listen for both events but only expect one of them, not both.

• Demo for both listeners present
• Demo for only mousewheel listener present