Add range selection ability on list contexts

This adds range select ability in two ways:
1) Sticky: like what we already have with the staging view i.e. press v then use arrow keys
2) Non-sticky: where you just use shift+up/down to expand the range

The state machine works like this:
(no range, press 'v') -> sticky range
(no range, press arrow) -> no range
(no range, press shift+arrow) -> nonsticky range
(sticky range, press 'v') -> no range
(sticky range, press arrow) -> sticky range
(sticky range, press shift+arrow) -> nonsticky range
(nonsticky range, press 'v') -> no range
(nonsticky range, press arrow) -> no range
(nonsticky range, press shift+arrow) -> nonsticky range

pkg/utils/utils.go

@@ -50,6 +50,13 @@ func Max(x, y int) int {
 	return y
 }
 
+func MinMax(x int, y int) (int, int) {
+	if x < y {
+		return x, y
+	}
+	return y, x
+}
+
 func Clamp(x int, min int, max int) int {
 	if x < min {
 		return min