title: Webpack 5 and support for UNC paths as output?
body: <p>If I set the <code>output.path</code> property of my webpack config to a UNC path like <code>\\COMPUTER-NAME\some-share</code> I get the following error since webpack 5 (It used to work in webpack 4):</p>
  <pre><code>C:\Temp\dbp\node_modules\webpack\lib\util\fs.js:121
                  throw new Error(
                  ^
  Error: \\COMPUTER-NAME\some-share is neither a posix nor a windows path, and there is no 'join' method defined in the file system
      at join (C:\Temp\dbp\node_modules\webpack\lib\util\fs.js:121:9)
      at C:\Temp\dbp\node_modules\webpack\lib\Compiler.js:790:31
      at arrayIterator (C:\Temp\dbp\node_modules\neo-async\async.js:3467:9)
      at timesSync (C:\Temp\dbp\node_modules\neo-async\async.js:2297:7)
      at Object.eachLimit (C:\Temp\dbp\node_modules\neo-async\async.js:3463:5)
      at emitFiles (C:\Temp\dbp\node_modules\webpack\lib\Compiler.js:536:13)
      at C:\Temp\dbp\node_modules\webpack\lib\util\fs.js:182:5
      at FSReqCallback.oncomplete (fs.js:171:23)
  </code></pre>
  <p>I looked in to the join method if <code>node_modules\webpack\lib\util\fs.js:121:9</code> and it looks like:</p>
  <pre><code>
  /**
   * @param {InputFileSystem|OutputFileSystem|undefined} fs a file system
   * @param {string} rootPath a path
   * @param {string} filename a filename
   * @returns {string} the joined path
   */
  const join = (fs, rootPath, filename) =&gt; {
      if (fs &amp;&amp; fs.join) {
          return fs.join(rootPath, filename);
      } else if (rootPath.startsWith(&quot;/&quot;)) {
          return path.posix.join(rootPath, filename);
      } else if (rootPath.length &gt; 1 &amp;&amp; rootPath[1] === &quot;:&quot;) {
          return path.win32.join(rootPath, filename);
      } else {
          throw new Error(
              `${rootPath} is neither a posix nor a windows path, and there is no 'join' method defined in the file system`
          );
      }
  };
  </code></pre>
  <p>From what I can tell here it seems like if the input fs does not have a join method a UNC path will not work.</p>
  <p>I'm not sure what the input fs params will be. I'm testing this on a windows system and I guess this function is not there...</p>
  <p>Should UNC paths be supported?</p>