fix: ensure context is consistent on concurrent match loads

[freshgiammi]

If a match has multiple load routines running (such as navigation happening while preload isn't finished yet), we might start the loader function too early when the context isn't ready for consumption yet.

This happens because execution flow is skipped when a route already in beforeLoad fetching state is re-loaded.

This patch adds beforeLoad tracking to matched routes, so that we can eventually await any pending beforeLoad function and ensure we have up to date context to feed into the loader function.

Fixes #1886

[nx-cloud]

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• nx run-many --target=build --exclude=examples/**

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[freshgiammi]

Weirdly enough, doing

matches[index] = match = updateMatch(
  match.id,
  (prev) => ({
    ...prev,
    loaderPromise: route.options.loader?.(loaderContext),
   }),
)

instead of this breaks navigate from working 🤔

[schiller-manuel]

closing in favor of #1907
I copied the test out of this PR, so thanks for this!