In JS, aliasing `exports = {}` omits the expected string index signature

[sandersn]

From adonis-framework in the user tests.

// Filename: util.js
const util = exports = module.exports = {}
util.existy = function() { }

Expected behavior:

No error, and import('util') has a property existy. (Ideally util would also have a property existy inside util.js, but this is not true in 2.9.2 either.)

Actual behavior:

Error: Property 'x' does not exist on type '{}', but require('util') does have a property existy: () => void.

[sandersn]

#25619 causes module.exports assignments to be contextually typed, and we don't add an index signature when there is a contextual type.

This wouldn't be a problem, except that util wasn't really the type of the module anyway, so without the index signature, existy didn't really exist.