compiler,std: implement ZON support

This commit allows using ZON (Zig Object Notation) in a few ways.

* `@import` can be used to load ZON at comptime and convert it to a
  normal Zig value. In this case, `@import` must have a result type.
* `std.zon.parse` can be used to parse ZON at runtime, akin to the
  parsing logic in `std.json`.
* `std.zon.stringify` can be used to convert arbitrary data structures
  to ZON at runtime, again akin to `std.json`.

lib/std/std.zig

@@ -93,6 +93,7 @@ pub const valgrind = @import("valgrind.zig");
 pub const wasm = @import("wasm.zig");
 pub const zig = @import("zig.zig");
 pub const zip = @import("zip.zig");
+pub const zon = @import("zon.zig");
 pub const start = @import("start.zig");
 
 const root = @import("root");