arrays: add a generic arrays.concat(os.args,'abc','xyz') function (#11985)

vlib/arrays/arrays.v

@@ -280,3 +280,13 @@ pub fn group_by<K, V>(list []V, grouping_op fn (v V) K) map[K][]V {
 
 	return result
 }
+
+// concatenate two arrays
+pub fn concat<T>(a []T, b ...T) []T {
+	mut m := []T{cap: a.len + b.len}
+
+	m << a
+	m << b
+
+	return m
+}