Enable (or fix) linting auto-formatting

[gavinr]

Our build process checks for linting errors, 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 :-)

See:

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

[patrickarlt]

This should end up being fairly similar to what we do for the other plugins and integrate Prettier and ESlint like on Esri/esri-leaflet#1408.