Default Tesla.Middleware.JSON engine to JSON module for Elixir 1.18

[Wigny]

Currently Tesla.Middleware.JSON defaults to Jason as its JSON decoder and encoder, but since Elixir 1.18 now there is a built-in alternative that could be used as the default engine when running in Elixit 1.18, eliminating a dependency on another library.

Right now, to use the new module, I believe we could use the encode and decode options, but it cannot be directly passed in the engine option since JSON does not have an encode/1 function and also do not support engine_opts.

{Tesla.Middleware.JSON, encode: &{:ok, JSON.encode!(&1)}, decode: &JSON.decode/1 }