[turbopack] Rename `rscEndpoint` to `rscHmrEndpoint` (#95538)
I'm introducing a new endpoint that doesn't compile SSR when doing a
page navigation, I'd like to call it a `rscEndpoint` but currently this
name is taken. `rscEndpoint` appears to be used for detecting changes
for HMR so I've renamed it to `rscHmrEndpoint` so I can use that name
for the new `rscEndpoint`.
