Only send wheel mouse event when scrolling a line or more

[pfitzseb]

Fixes #3848.

Haven't added a test since all of the mouse event functionality seems to be untested and I'm not sure how to set up the necessary scaffolding.

[pfitzseb]

Re-using viewport's _wheelPartialScroll isn't technically correct, but shouldn't have any real consequences; the same implementation is used elsewhere too:

xterm.js/src/browser/Terminal.ts

Line 863 in 52044e9

const amount = this.viewport!.getLinesScrolled(ev);

[Tyriar]

Looks good, I agree it's not ideal we're using this from Viewport but as you say it's used elsewhere already.