fix(zip): Tolerate XS null terminated string limitation

[kriskowal]

We encounter problems when attempting to decode the Uint8Array [0, 0] into a "\x00\x00" string under XS due to a limitation of our TextDecoder shim that is based on the XS String.fromArrayBuffer function on that engine Moddable-OpenSource/moddable#644. This work-around maintains the easy values for values comparison of compression method magic numbers/strings by switching the internal representation from string to number.

[dckc]

No tests needed updating? That doesn't seem right. Please consider adding one.

[kriskowal]

For testing, we do have an integration test that covers this. I’ve considered writing a close unit test, but testing that close would only serve as a change detector. I think the ideal test would be one that failed before this change, which is to say, run the existing integration test except under XS. I’ve left a note to do that work after we’ve moved xnap here #739.