chore: refactor logic for ignoring warnings

[Rich-Harris]

Wanted to take a look at this before doing #11294 but for warnings.

It feels really weird that we need to pass the current path into warn(...) in order to extract svelte-ignore comments — this is exactly the sort of thing state is supposed to be used for. So this PR inverts things: we capture ignore comments on the way down rather than searching up for them when we encounter a warning.

There is a wrinkle: because we can't use context.visit(...) during the analysis phase, we need to attach the set of ignored codes directly to the node in question, which is a bit hacky. We have this restriction because of our visitor merging logic. I've felt for some time now that this approach introduces much more complexity than it reduces, and that we'd be much better off having a single visitor per node during the analysis phase. I'm trying very hard to resist the temptation to validate this hypothesis until later when there are fewer more pressing concerns.

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