Fix the doc test for WebAssetPlugin

[andriyDev]

Objective

• Fix cargo t -p bevy_asset --all-features.
  • Note: all-features do not run on CI!
• The doc tests for the WebAssetPlugin don't compile at all. There are bad imports and we don't even add the AssetPlugin.

Solution

• Add missing imports, and add missing plugins to the App.

Testing

• Ran cargo t -p bevy_asset --all-features and it passes now!

[kfc35]

Looks good!
I also ran cargo t -p bevy_asset --all-features locally with these changes and it works for me as well.