Small inconsistency in target strings

[GearsDatapacks]

In Gleam v1.6.0-rc-1, the following code is valid and compiles:

@target(js)
fn wibble() {
  todo
}

But this doesn't:

@external(js, "./wibble.mjs", "wobble")
fn wibble() -> Nil

The shorthands, js and erl are allowed in various places such as @target and the --target flag for gleam build and gleam run. However, they are not allowed in @external. Small point, but I would expect the same set of values to be accepted in all such places.

[GearsDatapacks]

This inconsistency occurs because the parsing of @target uses Target's FromStr implementation, but @external explicitly matches on the possible string values.

[lpil]

Let's deprecate the target one.

[giacomocavalieri]

If we're to deprecate it we could also make the formatter rewrite @target(js) to @target(javascript) and same for erl

[GearsDatapacks]

It already does!