Support top-level await

As of Safari 15, all major browsers will support top-level await. However it looks like Closure Compiler doesn't support this. (It also appears to be missing a specific flag to indicate module input or output, perhaps that is related.)

Tested with v20210601, in.js: `await null;`

Command line: `java -jar ./compiler.jar --js in.js --js_output_file out.js`

Observed result:
```
in.js:1:0: ERROR - [JSC_PARSE_ERROR] Parse error. 'await' used in a non-async function context
1| await null;
^

1 error(s), 0 warning(s)
```

Expected result: allow usage of top-level await.