Use "async" as a function identifier.

[tunz]

Describe the bug

async is a keyword, but also allowed as an identifier.
https://tc39.es/ecma262/#sec-keywords-and-reserved-words

Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, meta, of, set, and target.

But, currently, it raises an error when calling a function named async.

To Reproduce
Run the following code:

async();

Uncaught "SyntaxError": "expected token 'function', got '(' in async function declaration at line 1, col 6"

Expected behavior
It should be a reference error, or running ok if "async" function is defined.

Note that async should be recognized as a keyword in the following case.

let fn = async () => { };

Additional context
Mentioned in #2269