add important eslint rules from leaflet repository and fix resulting warnings

[BePo65]

My intention for this pr was to make this repo even better, but feel free to just ignore it 😄.

To make eslint rules more consistent with leaflet, this pr adds the rules from the leaflet repository that are handled as errors and fixes the resulting warnings,

Besides this it:

• updates eslint to the current version
• adds 'npx' to the package.json script for linting, to make it runnable on *nix and windows alike
• adds the package-lock.json to git, to make all users of this repo have the same dependencies installed
• adds a .gitignore file, to prevent the node_modules directory to find its way to git

[KristjanESPERANTO]

I suggested a more extensive approach adding prettier and stylelint with #115. What do you think of it?

It might also be a good option to merge your PR and then I rebase my PR to discuss the introduction of prettier and stylelint later?

[BePo65]

This pr is no longer necessary - I support pr #115 instead