chore: dedupe getLocator calls

[Rich-Harris]

While looking at #11585, I thought it'd be worth printing the element locations as well as the filenames. To do that, we need to use getLocator. We're already doing that in multiple places, quite unnecessarily, and I didn't want to add yet another one.

This PR introduces compiler state, which includes warnings, the current filename, and a locator that can be used in multiple places — during parsing, when hitting warnings and errors, and when generating those SSR messages.

Once this is merged, it'll be trivial to add element locations to #11585.

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