Move eslint "webextensions" environment to .eslintrc.json

[lucasrmendonca]

Now, all environments are listed in the same place, for consistency

Before, ESLint environments weren't consistent across the lib: node and es6 were documented in .eslintrc.json, but webextensions was being used as a comment at the top of index.js.

[eight04]

This is a bad change. Not all js files run in the same environment e.g. rollup.config.js.