Parser: improve error message when await is used in non-async functions or instead of async

From https://x.com/jamonholmgren/status/1741906729477226534?s=20

- Better error message when the programmer accidentally wrote `await () =>` instead of `async() =>`
- Better error message for `await` when the programmer forgot to declare the surrounding function as async. These two errors compound in a funny way.

Start by locating Hermes parser diagnostics for `await` and `async` function declarations, then reproduce the two cases described: `await () =>` and `await` inside a non-async function. Done means each case produces a clearer, specific error message and the relevant parser tests cover both patterns.

Written by the indexing model from the issue text.