[JS IR] Improve debug info for suspend functions

- Map generated explicit Unit returns to the closing brace of
  the original body
- Name the continuation parameter as `$completion` to match the JVM BE,
  and generate debug info for it (so that it appears in the 'names'
  array in sourcemaps)
- Don't generate debug info for coroutine instantiation ceremony
  (so that the user doesn't need to step in many times to get where they
  want)

#KT-46276

compiler/testData/debug/stepping/suspendFunWithLambdaParameter.kt

@@ -19,13 +19,8 @@ suspend fun box() {
 // test.kt:11 box
 
 // EXPECTATIONS JS_IR
-// test.kt:7 box
-// test.kt:7 <init>
-// test.kt:7 <init>
-// test.kt:7 box
-// test.kt:7 box
-// test.kt:7 box
 // test.kt:8 doResume
 // test.kt:4 foo
 // test.kt:4 foo
 // test.kt:9 box$lambda
+// test.kt:11 doResume