[BREAKING CHANGES] JSZip package upgrade

[tiholic]

Upgraded JSZip to latest version 3.5.0 as per upgrade guide

This resulted in API changes as few methods required to be converted as async

• Updated tests to use async/await wherever necessary
• README updated
• All tests passed

Pre release checklist

• Major Release version upgrade - add necessary release notes

[TrySound]

Perhaps idea files above can be dropped as well.

[kant2002]

1. This require us to go to version 2.0
2. I would try to no create churn with breaking changes/version changes too much. While 2.0 would be backed, can we keep documentation for 1.0 intact and have 2.0 examples in readme somewhere in separate section. Maybe as separate file.
3. Also I agree in principle with this changes and that it is desirable, but can we have current version of JSZip package with mocked definitions like

declare module 'jszip' {
  var x: any;
  export = x;
}

that way it give us time to think how make less disruptive changes. Maybe we can take inspiration from https://stackoverflow.com/questions/16586640/node-js-async-to-sync and provide backward compatible API + async alternatives.

[tiholic]

@kant2002 let me know what would you like to do with this PR. If you want to have a v2, then have it as a separate release? or maintain it under separate branch for now and do a release later!

[kant2002]

@tiholic I want you to show how can we have cake and eat it too. I believe we can achieve your goals without major changes in the API and everything will just works. I will keep it for reference, but eventually likely it would not be accepted.

[Apobbot]

bump. The release of v2 would be much appreciated, as the jszip2.6.1 has a CWE-29 vulnerability. Most likely the jszip team wont release a hotfix for a version of 2016 release date, so a shift to async API with new jszip and v2 of xlsx-template is preferred.

Thanks.

[kant2002]

I made a fork of old JSzip for now and apply security fixes. So this is in its current form not needed. If you have idea how to expose async API so I can gradually migrate without disrupting users, I would appreciate it