[compiler] Scaffold fixture library (#31148)

Scaffolds an empty library to test backwards compatibility with the
compiler enabled.
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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/31148).
* #31167
* #31166
* #31165
* #31164
* __->__ #31148
* #31168

compiler/fixtures/runtime-compat/README.md

@@ -0,0 +1 @@
+Reference library compiled with React Compiler.

compiler/fixtures/runtime-compat/lib/index.js

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+throw new Error('Not implemented yet');

compiler/fixtures/runtime-compat/lib/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "runtime-compat-lib",
+  "version": "0.0.0",
+  "description": "Testing ground for libraries compiled with React Compiler",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo 'no tests'"
+  },
+  "license": "MIT",
+  "devDependencies": {
+    "@babel/cli": "^7.25.7",
+    "@babel/core": "^7.25.7",
+    "@babel/preset-env": "^7.25.7",
+    "babel-plugin-react-compiler": "0.0.0-experimental-58c2b1c-20241009"
+  },
+  "dependencies": {
+    "react-compiler-runtime": "0.0.0-experimental-8d8e73f-20241009"
+  }
+}