time: add custom formatter (#14202)

vlib/time/custom_format_test.v

@@ -0,0 +1,11 @@
+import time
+
+fn test_custom_format() {
+	date := time.now()
+	assert date.custom_format('YYYY-MM-DD HH:mm') == date.format()
+	assert date.custom_format('MMM') == date.smonth()
+
+	test_str := 'M MM MMM MMMM\nD DD DDD DDDD\nd dd ddd dddd\nYY YYYY a A\nH HH h hh k kk e\nm mm s ss Z ZZ ZZZ\nDo DDDo Q Qo QQ\nN NN w wo ww\nM/D/YYYY N-HH:mm:ss Qo?a'
+
+	println(date.custom_format(test_str))
+}