7$: Improve error message for 'await () => {}'

[strager]

Often, I accidentally type 'await' instead of 'async' for my async arrow functions:

function g() {
  f(await () => {
    await x;
  });
}

quick-lint-js reports errors for both await( and await x:

hello.js:3:5: error: 'await' is only allowed in async functions [E162]
hello.js:2:5: error: missing operator between expression and arrow function [E063]
hello.js:2:5: warning: use of undeclared variable: await [E057]

quick-lint-js should infer that async was intended, and provide a better error message (see below) and better error recovery (by assuming that the arrow function is async):

hello.js:2:5: error: 'await' does not create an async arrow function; use 'async' instead [E???]

[Coldtrain]

I claim this for-hire task. I expect payment after I complete this task. I will email the quick-lint-js team if I am assigned this task.

[strager]

This seems to be implemented already as E0178.