fix: detect memfs used by webpack-dev-server 4+

Webpack-dev-server 4+ uses memfs instead of memory-fs for in-memory
filesystem. The memfs library creates a filesystem object with a __vol
property that references the underlying Volume instance.

This fix checks for the __vol property to detect memfs and returns null
for bundleDir, which prevents the analyzer from trying to read bundle
files from disk when they only exist in memory.

Author: root

src/BundleAnalyzerPlugin.js

@@ -229,9 +229,17 @@ class BundleAnalyzerPlugin {
   }
 
   getBundleDirFromCompiler() {
-    const outputFileSystemConstructor =
+    const outputFileSystem =
       /** @type {OutputFileSystem} */
-      (/** @type {Compiler} */ (this.compiler).outputFileSystem).constructor;
+      (/** @type {Compiler} */ (this.compiler).outputFileSystem);
+
+    // Detect `memfs` (used by webpack-dev-server 4+) by checking for the `__vol` property
+    // Related: #471
+    if (/** @type {{ __vol?: unknown }} */ (outputFileSystem).__vol) {
+      return null;
+    }
+
+    const outputFileSystemConstructor = outputFileSystem.constructor;
 
     if (typeof outputFileSystemConstructor === "undefined") {
       return /** @type {Compiler} */ (this.compiler).outputPath;