Enable devtool: "source-map" in examples/utils/bundle-size-tests/webp…

…ack.config.cjs (microsoft#22711)

## Description

This enables another approach, source-map-explorer, for investigating
bundle size which gives more detailed and precise results.

This change was extracted from
microsoft#21336

examples/utils/bundle-size-tests/webpack.config.cjs

@@ -130,4 +130,7 @@ module.exports = {
 			file: path.resolve(process.cwd(), "bundleAnalysis/bundleStats.msp.gz"),
 		}),
 	],
+	// Enabling source maps allows using source-map-explorer to investigate bundle contents,
+	// which provides more fine grained details than BundleAnalyzerPlugin, so its nice for manual investigations.
+	devtool: "source-map",
 };