Show injected custom 404 route in dev (#6940)

* Add unit test for injected 404 routes

* Add fixture test for injected 404 routes

* Use any route pattern to find custom 404

* Fix frozen lockfile error

* Use `route` instead of `pattern` to match custom 404

Dynamic catch-all routes can match against `/404/` but will then error because they’re not actually designed to handle a param of `'404'`. Testing `route` instead excludes dynamic routes (because they’ll contain dynamic segments inside square brackets). Not sure if someone might _want_ to render the 404 via a dynamic route, but we already don’t support that, so this doesn’t change anything.

* Fix injected 404 fixture

* Add tests for `src/pages/404.html`

* Add changeset

* Fix lockfile

* And again

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