move indexOf into the vbantxt package.

improve the warning message on invalid bps value

util.go

@@ -0,0 +1,10 @@
+package vbantxt
+
+func indexOf[T comparable](collection []T, e T) int {
+	for i, x := range collection {
+		if x == e {
+			return i
+		}
+	}
+	return -1
+}