fix(viewport): normalize method names (#763)

* Reapply "fix(viewport): normalize method names"

This reverts commit f2434c374b.

* test: fixes

* fix: lint

* fix: disable horizontal scroll on v1

it might be a breaking change.

let's enable it by default on v2, and remove the deprecated methods as
well.

cc/ @meowgorithm

* fix: simplify

textarea/textarea_test.go

@@ -43,7 +43,7 @@ func TestVerticalScrolling(t *testing.T) {
 		"the text area.",
 	}
 	for _, line := range lines {
-		textarea.viewport.LineDown(1)
+		textarea.viewport.ScrollDown(1)
 		view = textarea.View()
 		if !strings.Contains(view, line) {
 			t.Log(view)