Support top-level await

[AshleyScirra]

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.

[rishipal]

AFAICT, changing the way modules load would require some pretty invasive changes to our broader internal JS stack, and not just the Closure compiler.

[ChadKillingsworth]

Top level await requires either the output to be a module OR the output to have an async IIFE. We should be able to support this when the chunk type is ES_MODULE, but we need a new flag otherwise.