fix: coordinate mount of snippets with await expressions

[Rich-Harris]

ok so i really borked up the git history in #17020. trying again. though it looks like some of the tests are failing

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pnpm add https://pkg.pr.new/svelte@17021

[Rich-Harris]

the test failures are something to do with custom elements. imagine my shock.

[dummdidumm]

Nice, this also helps with #16812 a bit (this PR does at least not pop in the inner snippet while the outer is still there)

[dummdidumm]

Oh wait this even fixed #16812 completely - it's just that in the reproduction the {@const ... } is turned into a blocking await and therefore both run at the same time.