Add support for Safari 14

[ggrossetie]

The latest version of Safari (version 14) support Web Extensions API:

Safari web extensions are available in macOS 11 and later, and in macOS 10.14.6 or 10.15.6 with Safari 14 installed.

Since Safari web extensions are implemented as macOS app extensions. We must use Xcode to package our extension and we must be a member of the Apple Developer Program to distribute the extension through the App Store.

In other words, Apple requires a yearly developer subscription of 99€ to distribute the extension through the App Store.
One community member (Michael Baker) has express interest and contributed financially through Open Collective to cover the yearly subscription expense.

References:

• https://developer.apple.com/documentation/safariservices/safari_web_extensions
• https://developer.apple.com/documentation/safariservices/safari_web_extensions/assessing_your_safari_web_extension_s_browser_compatibility
• https://hacks.mozilla.org/2020/06/welcoming-safari-to-the-webextensions-community/

[ggrossetie]

The extension is now available on the Apple Store

The code source is currently "duplicated" and available at: https://github.com/Mogztter/asciidoctor-safari-extension

Xcode and Safari extension have a somehow strict directory structure but I will try to merge everything into this repository.