Migrate `WebView` to `WKWebview`

According to [documentation](https://developer.apple.com/documentation/webkit/webview?language=objc), `WebView` is removed from Catalina, even though for some reason MacDown still works for me. I think we need to migrate as soon as possible.

Start by locating MacDown's current WebView usage and read Apple's linked WebView documentation alongside the project's macOS setup. Trace the existing web-view integration before assessing the WKWebView migration, then verify that the editor still renders Markdown and preserves current behavior on a supported macOS version.

Written by the indexing model from the issue text.