use ESLint config to determine parse options

https://github.com/eslint/eslint/blob/master/lib/config.js

Instead of relying fully on [UAJSG] to determine parsing goal, could also use ESLint config to get a parse goal.

If a file is marked `sourceType: module` by ESLint config, it will be assumed to be a module without disambiguation.

If it is marked `script`, disambiguation must be used as this should default to `script` for files outside ESLint's purview (i.e. `node_modules`, etc.) and thus can't be considered an _explicit_ parse goal.

For packages including `jsnext:main`, this is also ideal as the other parse options (parser, `experimentalObjectRestSpread`, `jsx`, etc.) can be leveraged from their (hopefully included and existing) `.eslintrc.*`.

Start with lib/config.js and trace how UAJSG currently determines parsing goals. Review how ESLint configuration exposes sourceType and parser options, including .eslintrc.* files. Done means configured modules avoid disambiguation, configured scripts retain it, and applicable parser options are honored.

Written by the indexing model from the issue text.