Add experimental touchstart flag for testing (#61747)
Adds an experimental flag to allow us to validate `touchstart` handling
x-ref: [slack
thread](https://vercel.slack.com/archives/C03S8ED1DKM/p1707269419316909?thread_ts=1707256828.309319&cid=C03S8ED1DKM)
Closes NEXT-2384
