Deprecate Js_json's serializeExn and deserializeUnsafe

[ryyppy]

Json.serializeExn and Json.deserializeUnsafe were introduced to allow string serialization of records that contain option values.

E.g.

let user = {
  name: "test",
  age: None
}

would compile to { name: "test", age: undefined} — a JS object that's not stringifiable due to the undefined value (which is not part of the JSON spec).

This was particularly an issue in Next, when using specific features like SSR to inject initial props (see #4443 for the full discussion).

Now with our new @optional feature (#5423) this is not needed anymore, since I would just declare a prop type as followed:

type user = {
  name: string,
 @optional age: int
}

Later on I'd just omit the age and be good to go. No undefined values in the output:

let user = {name: "patrick"}

// Equal to `{name: "patrick"}` in JS

My suggestion is to add a deprecation notice and point to the new docs of the @optional feature instead. Any opinions?

[ryyppy]

After some discussions with @cristianoc, it's probably better to wait a little longer until we move on with this. Will still keep it around for later reference.