Replace Rollup with Microbundle and create module/modern versions

[ocean90]

This PR is an alternative for #45. It uses Microbundle, a "zero-configuration bundler for tiny modules". I'd call Tobii a "tiny" module so this seems a perfect fit. 😀

Summary of the changes:

• Removes all rollup configs and dependencies
• Adds Microbundle as new dependency
• Renames src/js/tobii.js to src/js/index.js and removes all browser related imports (polyfill and CSS)
• Adds src/js/browser.js which imports all browser related files and src/js/index.js
• Updates the build commands to create all relevant files, see dist directory.
• Adds two demo HTML pages for the tobii.min.js and tobii.modern.js. Run npx serve . in the root folder and access them via http://localhost:5000/demo/ and http://localhost:5000/demo/modern

The config is inspired by Preact's config.

In my testing everything seems to continue to work as expected. I think this is ready and a much easier solution than #45. @midzer Let me know what you think.

[midzer]

Hi @ocean90 and many thanks for your PR.

I've never heard of Microbundle before, but this looks great! I did some additional testing on one of my projects using your forked branch and everything is fine.

I am going to merge this now :)