chore: build/transpile itowns to specific targets.

[gchoqueux]

Description

With babel it's possible to transpile itowns to specific targets. (visiting). I would like target to specific browser using browserslist.

Questions

• For example, the possible targets could be browsers supporting ES6.
• In webpack.config.js there is this comment (see bellow), I think it's obsolete, then we could remove target option { modules: false }, WDYT?

/*
   configuring babel:
   - when babel runs alone (for `test-unit` for instance), we let him deal with
   ES6 modules, because node doesn't support them yet (planned for v10 lts).
   - however, webpack also has ES6 module support and these 2 don't play well
   together. When running webpack (either `build` or `start` script), we prefer
   to rely on webpack loaders (much more powerful and gives more possibilities),
   so let's disable modules for babel here.
*/