feat: replace scroll-touch* with generic input-event

[nornagon]

Description of Change

This adds a new event to WebContents, input-event, that exposes the
underlying WebInputEvent stream.

As written, this only outputs a generic "WebInputEvent" unless the event is a
keyboard event, in which case it outputs some more information. Ultimately we
should expose all information available for each type of WebInputEvent.

This supercedes the old scroll-touch-edge and scroll-touch-end events, and
replaces scroll-touch-begin with a similar but not quite identical event.
Specifically, scroll-touch-begin used to be emitted when the user placed two
fingers down on the trackpad but before the touches had moved. Now it is only
emitted once the scroll gesture starts happening.

This somewhat overlaps with the before-input-event event on WebContents,
which is similar but only exposes keyboard events, and allows preventing the
event from being processed, which this API does not.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes:

• Added WebContents input-event event.
• Deprecated BrowserWindow scroll-touch-* events.

[nornagon]

i fixed the tests to use rawKeyDown instead of keyDown since before-input-event doesn't trigger for keyDown ... but i'm not sure that's the best approach. i think that breaks existing users of sendInputEvent.

[itsananderson]

API LGTM

[codebytere]

API LGTM

[release-clerk]

Release Notes Persisted

• Added WebContents input-event event.
• Deprecated BrowserWindow scroll-touch-* events.