Clean up project warnings (#199)

• Set "recommended project settings" for Mac app

• Add default fallback for font typingAttribute

• Clean up linter errors

• Update extension iconset

• Fix spm errors and upgrade sparkle (#203)

• Switch WriteFreely package import from ssh to https

• Sign macOS project target for Development

This fixes a crash-on-launch bug when running Product > Run from Xcode (Product > Archive should work correctly). See https://github.com/sparkle-project/Sparkle/issues/2056 for more details.

Bug introduced by commit:
671b0540ac526ca768c163c873ba32ecf01d7210

• Update Sparkle to v2.0 from official repo

Previously, we forked the repo to the WriteFreely org because Sparkle didn't support tag-based SPM updates. We therefore had to pull from the master branch, so forking insulated us from unexpected changes.

As of 2.x Sparkle now allows for tag-based updating via SPM; we'll allow patch updates but stop short of 2.1 until we can vet changes.

• Add MacUpdatesViewModel ObservableObject

• Use MacUpdatesViewModel to handle Sparkle update logic