address nodejs unit testing issue

mocha tests started failing as of node 20.10.0, likely due to this change:
https://github.com/nodejs/node/pull/49936

Error was:
node:internal/deps/undici/undici:11730
    Error.captureStackTrace(err, this);
          ^

TypeError: Failed to parse URL from js/zlib-1.2.13.wasm
    at Object.fetch (node:internal/deps/undici/undici:11730:11)
    at async initialize (/home/runner/work/PrivateBin/PrivateBin/js/zlib-1.2.13.js:31:26) {
  [cause]: TypeError: Invalid URL: js/zlib-1.2.13.wasm
      at new URLImpl (/home/runner/work/PrivateBin/PrivateBin/js/node_modules/jsdom-url/node_modules/whatwg-url/lib/URL-impl.js:21:13)
      at new URLImplCore (/home/runner/work/PrivateBin/PrivateBin/js/node_modules/jsdom-url/lib/URLImpl.js:18:9)
      at new URLCore (/home/runner/work/PrivateBin/PrivateBin/js/node_modules/jsdom-url/lib/URL.js:28:9)
      at Object.construct (/home/runner/work/PrivateBin/PrivateBin/js/node_modules/class-proxy/index.js:18:23)
      at new Request (node:internal/deps/undici/undici:5270:25)
      at fetch (node:internal/deps/undici/undici:9508:25)
      at Object.fetch (node:internal/deps/undici/undici:11728:18)
      at fetch (node:internal/process/pre_execution:314:27)
      at initialize (/home/runner/work/PrivateBin/PrivateBin/js/zlib-1.2.13.js:31:32)
      at Object.<anonymous> (/home/runner/work/PrivateBin/PrivateBin/js/zlib-1.2.13.js:145:17)
      at Object.<anonymous> (/home/runner/work/PrivateBin/PrivateBin/js/zlib-1.2.13.js:146:4)
[...]

Notice that the error occurs on line 31, meaning that fetch is not
undefined anymore. Node works on supporting fetch, which would make our
workaround using fs.readFileSync obsolete, but it (or rather the undici
library) currently doesn't support relative URLs.

js/zlib-1.2.13.js

@@ -25,7 +25,7 @@
         };
 
         let buff;
-        if (typeof fetch === 'undefined') {
+        if (typeof fs === 'object') {
             buff = fs.readFileSync('zlib-1.2.13.wasm');
         } else {
             const resp = await fetch('js/zlib-1.2.13.wasm');