Add `now` method and test

src/main.zig

@@ -395,6 +395,10 @@ pub const DateTime = struct {
         }
     }
 
+    pub fn now() Self {
+        return Self.fromTimestamp(std.time.timestamp());
+    }
+
     pub fn weekday(self: Self) WeekDay {
         const ts = self.toTimestamp();
         const days = @divTrunc(ts, SECONDS_PER_DAY);
@@ -471,6 +475,18 @@ test "conversions" {
     debug.print("Passed\n", .{});
 }
 
+test "now" {
+    const ts = std.time.timestamp();
+    const dt = DateTime.now();
+    try testing.expect(ts <= dt.toTimestamp());
+    // Make sure they're identical at least to the tens place, since it's possible
+    // for them to vary by a second or more depending on system resources
+    const a = @divTrunc(ts, 10);
+    const b = @divTrunc(dt.toTimestamp(), 10);
+    try testing.expectEqual(a, b);
+    debug.print("Passed\n", .{});
+}
+
 test "get weekday" {
     const dt = DateTime{
         .year = Year.new(2023),