Clean up/optimize @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?
• Only 1 is actually documented.

[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.