* Add eslint plus very tolerant starting config
Addresses GH #1865
This adds eslint as a developer dependency, plus a generated eslint
config that doesn't take a very strong stance on much of anything. The
goal here to get started using automated style checking, add eslint
checking to the testing flow, and gradually introduce stricter checks
over time.
* eslint: Fix ecmaVersion
See
https://github.com/Jermolene/TiddlyWiki5/pull/4421#issuecomment-587002325,
where @Jermolene declared TiddlyWiki targets EcmaScript 5
