JavaScript: Add support for top-level await.

[xiemaisi]

Evaluation shows a number of syntax errors going away in the test suites of babel, test262, gecko-dev and ChakraCore. There is also a (smaller) number of new syntax errors in tests exercising various tricky cases where await is used as a normal identifier, but I don't think it's worth bothering with fixing those. There are no changes at all outside test suites.

[asger-semmle]

There is also a (smaller) number of new syntax errors in tests exercising various tricky cases where await is used as a normal identifier

OOI could you share an example of this?

[xiemaisi]

Certainly: https://github.com/babel/babel/blob/0ee2c42b55e1893f0ae6510916405eb273587844/packages/babel-parser/test/fixtures/es2015/uncategorised/356/input.js#L

Technically, the proposal is to allow top-level await in modules. Presumably this test case is meant to be a stand-alone script, not a module, so await is not treated specially.

The parser only has a fairly vague idea of whether it is in a module or not, so I thought it best to always treat await as a keyword.

[asger-semmle]

Thanks. LGTM