Enable (or fix) linting auto-formatting

Our [build process checks for linting errors](https://github.com/Esri/esri-leaflet-geocoder/blob/059a831609a239cf568a31cbff8d43028e1a0e3b/package.json#L76), but code editor plugins typically will rely on an `.eslintrc` file to auto-lint for you. We should set this up so editing is easier in the future ( the [frustration is real](https://github.com/Esri/esri-leaflet-geocoder/pull/245#issuecomment-632052027) :-)

See:
- https://www.npmjs.com/package/eslint-config-semistandard
- https://eslint.org/docs/developer-guide/shareable-configs

Start with the linting command in package.json and review the referenced eslint-config-semistandard and shareable-config documentation. Determine the appropriate .eslintrc setup for this JavaScript project, then verify that editor linting and the existing build lint check use the same configuration.

Written by the indexing model from the issue text.