Remove @return(undefined_to_opt) etc.?

[cknitt]

We currently have the following special directives for external function definitions:

@return(nullable)
external someFunc1: unit => option<string> = "someFunc"

@return(null_undefined_to_opt)
external someFunc2: unit => option<string> = "someFunc"

@return(undefined_to_opt)
external someFunc3: unit => option<string> = "someFunc"

@return(null_to_opt)
external someFunc4: unit => option<string> = "someFunc"

• 1 and 2 are actually the same.
• For 2, 3, and 4, the naming is unidiomatic (no camelCase, ...).
• 3 is probably not needed anymore as one could just define the external to return an option.
• 4 would be for a case where we know that the external function can return null, but not undefined. But in that case using 1 would work as well?

I would suggest to deprecate and eventually remove everything but @return(nullable) (which is actually the only one that's documented BTW).

[cknitt]

Actually @return(undefined_to_opt) is useful to ensure nested options work correctly, see also the discussion in #7054 and #7430.

It is currently used in Js.Array(2) for example.