Label React syncs in GitHub

by keeping the version in a dedicated file that we can use to label PRs via `labeler.json`.

Makes it easier to search for React syncs in GitHub.

Since PRs are created manually, using the PR title for searching may not work all the time.
There's still a possibility that people don't use the sync script but why would you do that?
Even on manual updates they may stumble over `.react-version` and update it.

.github/labeler.json

@@ -70,6 +70,7 @@
       "packages/next-swc/**",
       "packages/next/**",
       "packages/react-refresh-utils/**"
-    ]
+    ],
+    "type: react-sync": [".react-version"]
   }
 }

.react-version

@@ -0,0 +1 @@
+19.0.0-rc-a26e3f403e-20240611

scripts/sync-react.js

@@ -177,6 +177,8 @@ Or run this command again without the --no-install flag to do both automatically
     )
   }
 
+  await fsp.writeFile(path.join(cwd, '.react-version'), newVersionStr)
+
   console.log(
     `Successfully updated React from ${baseSha} to ${newSha}.\n` +
       `Don't forget to find & replace all references to the React version '${baseVersionStr}' with '${newVersionStr}':\n` +