[DevTools] fix local build for extension (#26067)

## Summary

resolves #26051

After we upgrade to Manifest V3, the browser no longer allow us to run
`eval` within the extension. It's not a problem for prod build, but for
dev build, webpack has been using eval to inject the source map for
devtool. This PR changes it to an alternative method.

Author: mondaychen

packages/react-devtools-extensions/webpack.backend.js

@@ -36,7 +36,7 @@ const featureFlagTarget = process.env.FEATURE_FLAG_TARGET || 'extension-oss';
 
 module.exports = {
   mode: __DEV__ ? 'development' : 'production',
-  devtool: __DEV__ ? 'cheap-module-eval-source-map' : false,
+  devtool: __DEV__ ? 'cheap-module-source-map' : false,
   entry: {
     backend: './src/backend.js',
   },

packages/react-devtools-extensions/webpack.config.js

@@ -48,7 +48,7 @@ const babelOptions = {
 
 module.exports = {
   mode: __DEV__ ? 'development' : 'production',
-  devtool: __DEV__ ? 'cheap-module-eval-source-map' : false,
+  devtool: __DEV__ ? 'cheap-module-source-map' : false,
   entry: {
     background: './src/background.js',
     main: './src/main.js',