Defer client hooks import to allow using env in them

[gtm-nayan]

ref #8464

Need some insight on this commit, without it, all the tests start failing at random because it can't find the selector that's set on hydration, no matter what the timeout is. Although it seems like the wrong location to put it in.

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[dummdidumm]

It's weird indeed that this makes things fail, don't have a good idea why just from looking at it - before this change, it was loaded even earlier (eagerly inside client.js).

@dominikg you had ideas about some init hook that blocks starting the client until for example the language is determined. Would the timing of "create and init client -> run hooks -> hydrate and start router" be wrong for this use case?

[Rich-Harris]

going to close this in favour of #8957, which is finally inching towards being shippable, after a detour that involved learning more than i ever thought i'd need to know about the behaviour of preload and modulepreload links