feat: add server.hmr.update to allow configuring ws settings while disabling HMR

[sapphi-red]

Description

I noticed that it's not possible to configure the ws settings when the HMR is disabled by server.hmr: false. This PR adds server.hmr.update: false option which works similarly to server.hmr: false.

The alternative way is to move all ws related options under server.ws. This feels more correct, but I'm not sure if the required migration effort for the users is worth.

fixes #18489
refs #18489 (comment), #18489 (comment),

[bluwy]

Interesting. Yeah since we had issues being confused of hmr: false not actually disabling websockets, I think it's worth considering a new ws option instead, or rename as ws and put hmr into ws.hmr: boolean.

I think we can discuss more in the next meeting, but migration-wise I think it'll hurt for sure but we can bring the change slowly like, (1) deprecating via types first, (2) log deprecation message v8.1?, (3) remove in v9.