time: add is_leap_year(year int)

vlib/time/time_test.v

@@ -0,0 +1,17 @@
+import time
+
+fn test_is_leap_year() {
+    assert time.is_leap_year(1700) == false
+    assert time.is_leap_year(1800) == false
+    assert time.is_leap_year(1900) == false
+
+    assert time.is_leap_year(1600) == true
+    assert time.is_leap_year(2000) == true
+
+    assert time.is_leap_year(2100) == false
+    assert time.is_leap_year(2200) == false
+    assert time.is_leap_year(2300) == false
+
+    assert time.is_leap_year(1996) == true
+    assert time.is_leap_year(1997) == false
+}