[Flight] Enable Server Action Source Maps in flight-esm Fixture (#30763)

Stacked on #30758 and #30755.

This is copy paste from #30755 into the ESM package. We use the
`webpack-sources` package for the source map utility but it's not
actually dependent on Webpack itself. Could probably inline it in the
build.

Author: sebmarkbage

fixtures/flight-esm/package.json

@@ -13,14 +13,15 @@
     "prompts": "^2.4.2",
     "react": "experimental",
     "react-dom": "experimental",
-    "undici": "^5.20.0"
+    "undici": "^5.20.0",
+    "webpack-sources": "^3.2.0"
   },
   "scripts": {
     "predev": "cp -r ../../build/oss-experimental/* ./node_modules/",
     "prestart": "cp -r ../../build/oss-experimental/* ./node_modules/",
     "dev": "concurrently \"npm run dev:region\" \"npm run dev:global\"",
     "dev:global": "NODE_ENV=development BUILD_PATH=dist node server/global",
-    "dev:region": "NODE_ENV=development BUILD_PATH=dist nodemon --watch src --watch dist -- --experimental-loader ./loader/region.js --conditions=react-server server/region",
+    "dev:region": "NODE_ENV=development BUILD_PATH=dist nodemon --watch src --watch dist -- --enable-source-maps --experimental-loader ./loader/region.js --conditions=react-server server/region",
     "start": "concurrently \"npm run start:region\" \"npm run start:global\"",
     "start:global": "NODE_ENV=production node server/global",
     "start:region": "NODE_ENV=production node --experimental-loader ./loader/region.js --conditions=react-server server/region"

fixtures/flight-esm/yarn.lock

@@ -755,6 +755,11 @@ vary@~1.1.2:
   resolved "https://registry.yarnpkg.com/vary/-/vary-1.1.2.tgz#2299f02c6ded30d4a5961b0b9f74524a18f634fc"
   integrity sha512-BNGbWLfd0eUPabhkXUVm0j8uuvREyTh5ovRa/dyow/BqAbZJyC+5fU+IzQOzmAKzYqYRAISoRhdQr3eIZ/PXqg==
 
+webpack-sources@^3.2.0:
+  version "3.2.3"
+  resolved "https://registry.yarnpkg.com/webpack-sources/-/webpack-sources-3.2.3.tgz#2d4daab8451fd4b240cc27055ff6a0c2ccea0cde"
+  integrity sha512-/DyMEOrDgLKKIG0fmvtz+4dUX/3Ghozwgm6iPp8KRhvn+eQf9+Q7GWxVNMk3+uCPWfdXYC4ExGBckIXdFEfH1w==
+
 wrap-ansi@^7.0.0:
   version "7.0.0"
   resolved "https://registry.yarnpkg.com/wrap-ansi/-/wrap-ansi-7.0.0.tgz#67e145cff510a6a6984bdf1152911d69d2eb9e43"

packages/react-server-dom-esm/package.json

@@ -58,6 +58,7 @@
     "react-dom": "^19.0.0"
   },
   "dependencies": {
-    "acorn-loose": "^8.3.0"
+    "acorn-loose": "^8.3.0",
+    "webpack-sources": "^3.2.0"
   }
 }