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
+}

vlib/arrays/arrays_test.v

@@ -180,3 +180,17 @@ fn test_group_by() {
 		return 0
 	}) == map[int][]int{}
 }
+
+fn test_concat_int() {
+	mut a := [1, 2, 3]
+	mut b := [3, 2, 1]
+
+	assert concat(a, ...b) == [1, 2, 3, 3, 2, 1]
+}
+
+fn test_concat_string() {
+	mut a := ['1', '2', '3']
+	mut b := ['3', '2', '1']
+
+	assert concat(a, ...b) == ['1', '2', '3', '3', '2', '1']
+}