Build front end to support older webkit versions

[mattyg]

A major challenge with using Tauri for the launcher is many users will be stuck on older versions of webkit and we can't control it. Unless we get a version of launcher using electron down-the-road, this may be an issue we're stuck with.

What we can do is configure front-end builds to include polyfills that implement our necessary js features. This seems particularly import for any front-end modules that will be widely used in other happs.

• Tauri webview verisons: https://tauri.app/v1/references/webview-versions/
• Example of user having issues due to older webkit version fixed undefined holochain manager holochain/launcher#148

Seems like targeting Safari >= 13 would satisfy most users.