hltrailingws: improve updateTrailingWs logic

Handle the case when the cursor itself hasn't really moved to
another line, but its line number has changed due to insert
or remove of some lines above.
In this case, if the cursor is still at its new trailingws,
we should not reset NewTrailingWsY to -1 but update it to the
new line number.

A scenario exemplifying this issue:
Bind some key, e.g. Alt-r, to such a lua function:

function insertNewlineAbove(bp)
    bp.Buf:Insert(buffer.Loc(0, bp.Cursor.Y), "\n")
end

Then in a file containing these lines:

aaa
bbb
ccc

insert a space at the end of bbb line, and then press Alt-r.
bbb and ccc are moved one line down, but also the trailing space
after bbb becomes highlighted, which isn't what we expect.
This commit fixes that.

internal/buffer/eventhandler.go

@@ -391,5 +391,11 @@ func (eh *EventHandler) updateTrailingWs(t *TextEvent) {
 		} else if !removedAfterWs {
 			c.NewTrailingWsY = -1
 		}
+	} else if c.NewTrailingWsY != -1 && start.Y != end.Y && c.Loc.GreaterThan(start) &&
+		((t.EventType == TextEventInsert && c.Y == c.NewTrailingWsY+(end.Y-start.Y)) ||
+			(t.EventType == TextEventRemove && c.Y == c.NewTrailingWsY-(end.Y-start.Y))) {
+		// The cursor still has its new trailingws
+		// but its line number was shifted by insert or remove of lines above
+		c.NewTrailingWsY = c.Y
 	}
 }