Windows Webview without bundling `WebView2Loader`

[mochalins]

Rather than relying on users to provide and bundle WebView2Loader.dll or statically link WebView2Loader.lib, a custom WV2 loader implementation can be embedded in webui.

An existing embedded Windows WV2 loader is referenced below:

https://github.com/webview/webview/blob/master/core/include/webview/detail/platform/windows/webview2/loader.hh