Require capture annotation on extern definitions

Time and time again I and others have observed that capture annotations on externs defaulting to io is more confusing than always requiring it to be explicitly annotated. The following should no longer be accepted:

```
extern def alert(message: String): Unit =
js "alert(message)"
```

Instead one has to write the following with the same meaning:

```
extern def alert(message: String) at io: Unit =
js "alert(message)"
```

The issue names no files or tests. Start by searching the compiler for extern definitions and capture-annotation handling, then run the existing extern-related tests if found. Done means the unannotated form is rejected while the explicit `at io` form remains accepted.

Written by the indexing model from the issue text.