chore(integ-runner): add fsevents to optionalDependencies (#26151)

This matches the behavior of [aws-cdk](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk/package.json#L31-L35)

When `integ-runner` is installed on a mac, `fsevents` will be installed as well.

----

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packages/@aws-cdk/integ-runner/package.json

@@ -29,6 +29,9 @@
         "lib/workers/extract/index.js"
       ],
       "externals": {
+        "optionalDependencies": [
+          "fsevents"
+        ],
         "dependencies": [
           "aws-cdk"
         ]