macOS & iOS testing

[GeopJr]

macOS needs further testing on both SveltePress and SveltePress-native.

What should be done:

• [ ] - Test SveltePress dev & build
• [ ] - Test Flutter GUI (both macOS & iOS if possible)
• [ ] - Test QT GUI
• [ ] - Test GTK GUI (needs special libraries)
• [ ] - Test if Pandoc builds

Flutter, QT & GTK have feature boards that you can fill to make a PR. (Please test all of them)

To test local assets, append to any markdown post the following: ![](/android-chrome-512x512.png) (don't include it in the PR)

Note, for GUIs to work, you need to have built SveltePress in prod mode.