refactor: replace @target with external fallbacks

Author: jtdowney

src/unitest.gleam

@@ -1458,11 +1458,7 @@ fn start_module_pool(
   workers: Int,
 ) -> Nil
 
-@target(erlang)
+@external(javascript, "./unitest_ffi.mjs", "yieldThen")
 fn yield_then(next: fn() -> Nil) -> Nil {
   next()
 }
-
-@target(javascript)
-@external(javascript, "./unitest_ffi.mjs", "yieldThen")
-fn yield_then(next: fn() -> Nil) -> Nil

src/unitest/internal/discovery.gleam

@@ -11,11 +11,10 @@ const gleam_extension = ".gleam"
 
 const test_directory = "test"
 
-@target(erlang)
-const current_target = "erlang"
-
-@target(javascript)
-const current_target = "javascript"
+@external(javascript, "../../unitest_ffi.mjs", "currentTarget")
+fn current_target() -> String {
+  "erlang"
+}
 
 pub type LineSpan {
   LineSpan(start_line: Int, end_line: Int)
@@ -137,7 +136,7 @@ fn tests_from_contents(
 }
 
 pub fn parse_module(source: String) -> Result(List(ParsedTest), glance.Error) {
-  parse_module_for_target(source, current_target)
+  parse_module_for_target(source, current_target())
 }
 
 pub fn parse_module_for_target(

src/unitest_ffi.mjs

@@ -101,6 +101,10 @@ export function nowMs() {
   return Math.round(performance.now());
 }
 
+export function currentTarget() {
+  return "javascript";
+}
+
 export function halt(code) {
   if (runtime === "node") {
     process.exitCode = code;