TUN-3201: Create base cloudflared UI structure

vendor/github.com/rivo/tview/focusable.go

@@ -0,0 +1,8 @@
+package tview
+
+// Focusable provides a method which determines if a primitive has focus.
+// Composed primitives may be focused based on the focused state of their
+// contained primitives.
+type Focusable interface {
+	HasFocus() bool
+}