Add initial Date bindings

Doesn't do much, yet, because we can't get the timezone with a plain binding, but it's at least something.

std/assembly/bindings/Date.ts

@@ -0,0 +1,10 @@
+export declare function UTC(
+  year: i32,
+  month: i32,
+  day: i32,
+  hour: i32,
+  minute: i32,
+  second: i32,
+  millisecond: i32
+): f64;
+export declare function now(): f64;