It seems that at least some of the WebView2 source code may be available: https://3rdpartycodeprod.blob.core.windows.net/download/Microsoft%20Edge/136... (from https://thirdpartysource.microsoft.com/, search "microsoft edge"). A quick search shows some results for "webview2":
That said, if the full WebView2 source isn't present, I'm not sure it makes sense for Wine to ship a fake one or a partial implementation. Better to let the application see WebView2 is missing and install it as part of the installation process IMHO (just like modern .NET).