Rework AssetHandler (#870)

- Rename `AssetHandler` to `AssetReader` and move to `src/readers`
- Update `AssetReader` API to make clear what resources need to be made available to DWDS
- Rename `BuildRunnerAssetHandler` to `ProxyServerAssetReader`
- Break out `ProxyServerAssetReader` into `proxy_server_asset_reader.dart` and move to `src/readers`
  - Note in the future we will have `FrontendServerAssetReader` with similar functionality
- Move `readAssetOrNull` logic into `ProxyServerAssetReader` as it will likely not apply to the `FrontendServerAssetReader`
- Update `webdev` to adhere to the new API requirements

Towards https://github.com/dart-lang/webdev/issues/865
18 files changed