Multiline control externs broken by JS semicolon insertion

When the user defines a JS extern with control effects like so:
```effekt
extern control def foo(): Int =
js """
$effekt.pure(2)
"""
```
Then the call fails at runtime with an error like:
```
foo_2839().then((v_r_2852_3977) =>
^

TypeError: Cannot read properties of undefined (reading 'then')
at Object.head (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:828:15)
at apply (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:459:25)
at Object.apply (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:549:34)
at trampoline (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:435:19)
at Object.run (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:542:18)
at Object.main (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs.js:836:27)
at Object. (/Users/gaisseml/dev/effekt/experiments/out/js_multiline_externs:2:79)
at Module._compile (node:internal/modules/cjs/loader:1480:14)
at Module._extensions..js (node:internal/modules/cjs/loader:1564:10)
at Module.load (node:internal/modules/cjs/loader:1287:32)

Node.js v22.1.0
[error] Process exited with non-zero exit code 1.
```

## Expected behaviour
This should return 2.

## Why does this happen
The above definition gets translated to the following JS:
```js
function foo_2839() {
return
$effekt.pure(2)
;
}
```
Semicolon insertion will insert a `;` after the return, so this is equivalent to something like:
```js
function foo_2839() {
return undefined; // early return
$effekt.pure(2); // dead code
;
}
```

## Full example code
```effekt
extern control def foo(): Int =
js """
$effekt.pure(2)
"""

extern control def bar(): Int =
js """$effekt.pure(1)"""

def main() = {
println(bar())
println(foo())
}
```