Use an ESM-friendly UMD wrapper

[fuweichin]

Overview
The UMD wrapper js-untar currently in use may cause an error in certain circumstances.

Steps to Reproduce:

<script type="module">
import '/node_modules/jquery/dist/jquery.min.js';/* global jQuery */
import '/node_modules/pako/dist/pako_inflate.min.js';/* global pako */
import '/node_modules/js-untar/build/dist/untar.js';/* global untar */ 
//...
</script>

jquery and pako are okay with native ESM loader, while js-untar will throw an error

TypeError: Cannot set property 'untar' of undefined

Possible Solution:
Use a more versatile variant of UMD, e.g. returnExportsGlobal.js

[guest271314]

In the browser I get the error

TypeError: Cannot set properties of undefined (setting 'untar')
    at untar.js:1:118
    at untar.js:1:123

using https://unpkg.com/js-untar@2.0.0/build/dist/untar.js.

Currently I handle the error and continue

try {
  await import('https://unpkg.com/js-untar@2.0.0/build/dist/untar.js');
} catch(e) {
   console.log(e);
} finally {
  // do stuff
}

[guest271314]

The TypeError is thrown due to

js-untar/src/untar.js

Line 5 in 49e639c

var global = window || this;

this is not defined in Ecmascript Modules. Substituting globalThis avoids the error.