all: add missing copyright headers

Tests are CC0/public domain because there's no reason for them not to
be. Examples are also CC0/public domain, but this may be a little bit
weird because they are largely straightforward ports of examples from
nats.c which carry the Apache license. However, I personally wrote
them against the zig bindings and I doubt anyone will end up in a
court of law due to their software containing uselessly trivial
example code.

examples/headers.zig

@@ -1,3 +1,6 @@
+// This file is licensed under the CC0 1.0 license.
+// See: https://creativecommons.org/publicdomain/zero/1.0/legalcode
+
 const std = @import("std");
 const nats = @import("nats");
 

examples/pub_bytes.zig

@@ -1,3 +1,6 @@
+// This file is licensed under the CC0 1.0 license.
+// See: https://creativecommons.org/publicdomain/zero/1.0/legalcode
+
 const std = @import("std");
 const nats = @import("nats");
 

examples/request_reply.zig

@@ -1,3 +1,6 @@
+// This file is licensed under the CC0 1.0 license.
+// See: https://creativecommons.org/publicdomain/zero/1.0/legalcode
+
 const std = @import("std");
 const nats = @import("nats");