Typed alternative to `load_folder`

## What problem does this solve or what need does it fill?

For my game I'm loading a single file with two different asset loaders, each producing a separate asset. One asset is a `BlockDefinition` which will contain all properties necessary to simulate the block, such as its collision shape and behavior. The other asset is a `BlockVisuals` which holds all that is necessary to display the block, such as mesh, texture and material. The latter would not be loaded on a headless server.

I want to be able to hot reload these assets including adding and removing them on the fly for ease of development or modding.

## What solution would you like?

It would be nice to have a typed alternative to `load_folder` that allows me to specify what type of asset I want to load. Since this API will decide what type of asset to load a file as depending on the extension, I won't ever be able to load a single `.ron` file as two separate types of assets using it.

## What alternative(s) have you considered?

- Hardcode the asset paths instead.
- Use a manifest file which mentions all the other files that need to load. [Necessary on web anyway?](https://github.com/bevyengine/bevy/issues/10459)
- Split the file into two, with different extensions, so `load_folder` can handle them.
- Register only a single asset loader, but a different one depending on whether on headless or client, which can then load `BlockDefinition` and `BlockVisuals` as sub-assets/dependencies from the single file with the right type explicitly, and use asset events to collect them into resources? Seems a bit over-engineered.

## Additional context

Source code of the project available to browse [here](https://git.mcft.net/copygirl/bevy-bloxel-classic).

## Related issues / pull requests

- #19760 has the same issue. But a typed version of it would be nice.
- #11061 mentions how it's no longer possible to access directories. (Though unsure how this would work with hot reloading.)
- #2291 – OP mentions a typed API, but commenters ended up talking about filtering paths only.

Start with the existing `load_folder` API and compare the related discussions in issues #19760, #11061, and #2291. Define the typed loading behavior, including extension-based type selection and hot reloading, then confirm the API handles the stated single-file and headless-server use cases.

Written by the indexing model from the issue text.