Add more --border options

Instead of drawing the window border in Vim using an extra window,
extend the --border option so that we do can it natively.

Close #2223
Fix #2184

src/tui/tui.go

@@ -259,6 +259,11 @@ const (
 	BorderRounded
 	BorderSharp
 	BorderHorizontal
+	BorderVertical
+	BorderTop
+	BorderBottom
+	BorderLeft
+	BorderRight
 )
 
 type BorderStyle struct {