Prevent asset_path clashing with route definition in a rails app

[ollie-nye]

Closes rails/rails#44403

If a route is defined that generates the helper asset_path, the view_context passed to importmap.to_json will use the resource route instead of the asset pipeline route. By default, this is aliased to path_to_asset, so updating this method to use that instead resolves the issue.

[gmcgibbon]

This changes the contract of a resolver and is likely a breaking change, but I think it makes sense 👍

[matthewd]

Yeah, this is technically a breaking change of the expected API, but in practice to me it seems early enough in the library's lifetime that we can reasonably assume currently-extant callers are almost certainly passing a Rails-sourced object that already meets this criteria.

We could retain compatibility with a respond_to? check and a fallback, but it really just seems like avoidable complication.