Implement new polyfilling technique

[lukastaegert]

resolves #8

(I would also assume it resolves #4, but that would need verification I guess)

As promised, this PR implements the new polyfill approach I discussed. I also updated dependencies and implemented a comprehensive test suite.

The test setup may look slightly involved, I mostly copied and adapted it from the commonjs plugin. One may consider moving it to a separate file.

Other things to consider:

• Add proper CI to run tests automatically. As a first safety measure, the tests are now run before publishing.
• Add a linter/prettier

In any case, even though the plugin is now noticeably more complicated, it is now also noticeably simpler to use:

• Neither the method nor the sourceMap option are needed any more as we are no longer transforming files. Things should "just work"
• Without the use of magic-string, performance should likely improve a little
• We no longer need the magic-string dependency

Feel free to modify as you see fit.

[JRJurman]

@lukastaegert I did a final check, and everything looks good - one final request, if you could update the minimum version in the /example/package.json of rollup to whatever is required, then I think this will be perfect, and I can merge and publish this :+1:

[lukastaegert]

Done

[perrin4869]

@lukastaegert just one question, will sourcemaps still work with v4?

[lukastaegert]

Sure, as we are not transforming anything, there is nothing to mess up.