Add a feature to allow any json in a variant

[EmileTrotignon]

Syntax is as follow :

type t =
| Other of Yojson.Basic.t [@allow_any]
| Foo
[@@deriving json]

Then, if the json j is invalid for type t, Other j is returned instead.

There is support for both runtime and js (although their Json type is different: is that an issue ? We could introduce a [%json] extension that translates to Js.Json.t in Js and Yojson.Basic.t in runtime if its an issue.)

As a bonus, I improved an error message that slipped through in my error message PR. Because its only on JS I did not find it with my cram tests.

[EmileTrotignon]

I also moved the common folder inside the native one. The reason for that is that then I can use it with include subdirs, which enables merlin inside it, which is very useful for developing.

using copy_files means that merlin does not work inside these files.

[EmileTrotignon]

Something similar exists in atdgen, it only allows extra tags instead of any json. I think allowing any json is a good idea, especially because we have structured types for json so we can easily check afterward if its tag-shapped.

[anmonteiro]

I think this makes sense, no strong opinions

[EmileTrotignon]

Hey @anmonteiro I have added the label you asked, and remove the stray comment. Any other change required ?