Make dojo more ESM friendly #905

Enhancement
Currently we ship a bit of a mishmash of module formats. The default .js we ship is legacy ES5 and UMD. Then we also ship .mjs which is pseudo modern in that it has ESM style imports but not quite correct. Would like to propose some changes to make this work out the box better.

• The mjs is currently not picked up at all by ESM cdn's such as jspm. Do we need a package exports set up for ESM?
• Should we be appending file extensions to our ESM module names for in browser?
• Explore import maps and usage with framework
• Currently the static-build-loader in webpack-contrib takes our modules and makes optimisations for modern browsers at app build time. It would be good if we could apply these optimisations to the mjs for the actual package up front.