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`.

src/main.zig

@@ -6278,7 +6278,7 @@ fn cmdAstCheck(
             }
         },
         .zon => {
-            const zoir = try ZonGen.generate(gpa, file.tree);
+            const zoir = try ZonGen.generate(gpa, file.tree, .{});
             defer zoir.deinit(gpa);
 
             if (zoir.hasCompileErrors()) {