feat(test) react-native & @react-native/ packages version alignment

These have to be aligned, unless we're not on a stable branch.

scripts/__tests__/package_json-test.js

@@ -0,0 +1,34 @@
+const fs = require('fs');
+const {execSync} = require('child_process');
+
+const branch = execSync('git rev-parse --abbrev-ref HEAD').toString().trim();
+
+const isStableBranch = /\d+\.\d+-stable$/.test(branch);
+
+const isReactNativePackage = pkg => pkg === "react-native" || pkg.startsWith("@react-native/");
+
+const label = isStableBranch ? 'react-native and ' : '';
+
+describe("react-native packages on a version branch need to be aligned", () => {
+  it(`has a consistent version for ${label}@react-native/ scoped packages in the template/package.json`, () => {
+    const pkgJson = JSON.parse(fs.readFileSync('template/package.json'));
+
+    const everything = Object.entries({
+      ...pkgJson.dependencies,
+      ...pkgJson.devDependencies,
+      ...pkgJson.peerDependencies ?? {}
+    });
+
+    const versions = Object.fromEntries(everything.filter(([name]) => isReactNativePackage(name)));
+
+    if (!isStableBranch) {
+      // This is the one case where "react-native" doesn't have to match
+      delete versions["react-native"];
+    }
+
+    const allReactNativeVersions = new Set(Object.values(versions));
+
+    // Only a single version
+    expect(allReactNativeVersions.size).toEqual(1);
+  });
+});