Make the file engine feature in the html crate a no-op

[ealmloff]

This PR removes all usage of the the file engine feature flag in the html crate. This feature flag is useful to reduce the binary size of web builds which used to live in the html crate. Now that the bindings are moved to the web crate, we can remove the feature flag from dioxus-html.

This PR does not actually remove the feature flag because that would be a breaking change. This PR does enable a method on a trait when the file_engine trait is disabled, but that method has a default implementation so adding the method should be non-breaking.

I manually tested building the dioxus-web crate with the default features disabled and only the mounted, file_engine, or hydrate features enabled and all three builds succed.

Fixes #3383

[jkelleyrtp]

one day we can remove all the weird flags... 🫠