Upgrade jszip to 3.x to resolve CVE-2021-23413

[lux01]

Appmetrics currently uses [email protected] which has a prototype pollution vulnerability:

• CVE-2021-23413
• npm advisory 1774
• GitHub advisory GHSA-jg8v-48h5-wgxg

This PR upgrades the jszip dependency to ^3.7.0 and consequently tweaks the headless_zip.js functions to be more async friendly as zip.generate() has been replaced by zip.generateAsync().

This fixes #655.

Additionally I had to fix a unit test failure that occurs when running on macOS 11 (caused by os.name now returning macOS rather than Mac OS X) to ensure my changes still passed the unit tests.