PWA and Service workers: Use native Fetch API, remove Axios

[lbausch]

Progressive Web Apps (PWAs) utilize Service Workers which may be used to implement offline functionality for apps. This however only works for requests performed with the native Fetch API (see 1 and the Exposed attribute in the spec 2).

Since Inertia comes with Axios (and therefore uses XHR) it seems impossible to achieve offline functionality right now.
I'm aware of several closed PRs (#91, #29, #19) and the fact that the Fetch API does not support tracking upload progress at the moment. I've also looked at https://github.com/developit/redaxios but I'm not sure if and how easy it might be used with Inertia.

So, ultimately this discussion is another pitch to ditch XHR for Fetch. Maybe it is possible to use the Fetch API for all requests except uploads and fall back to XHR for those.

In any case, thank you for maintaining Inertia, I really appreciate your work!