Set babel runtime version to make smaller bundle

[anajavi]

This sets used runtime version for @babel/transform-runtime.
https://babeljs.io/docs/en/babel-plugin-transform-runtime#version

It results in a smaller bundle, as transform can depend on newer features be present on runtime.

Before:

97810 react-leaflet.js
42915 react-leaflet.min.js

After:

87280 react-leaflet.js
36061 react-leaflet.min.js

By using .babelrc.js, the version could come from package.json like this:

const ENV = process.env.BABEL_ENV || process.env.NODE_ENV || 'development';
const pkg = require('./package.json');
const runtimeVersion = pkg.dependencies['@babel/runtime'];

But probably the need for version field is removed in Babel 8 anyway.

[PaulLeCam]

Great idea, thanks!