Full Windows file paths cause a panic when loading assets

## Bevy version

dcc35120f955dd921149c1a9ed3b68bf3cec848e

## Relevant system information

Windows 11

## What you did

Run the scene viewer using a full windows file path (eg. `D:/path/to/scene.gltf/`) instead of a relative path.

## What went wrong

It panics with ```thread 'Compute Task Pool (15)' panicked at crates\bevy_asset\src\path.rs:115:70:
called `Result::unwrap()` on an `Err` value: InvalidSourceSyntax```

## Additional information

35073cf7aa9eaf748e167c103921a6764fcedfb1 is when the issue started. There was also some discussion on discord if `AssetPath` should support Windows file paths (https://discord.com/channels/691052431525675048/1164297864441249823)