SwiftUI / Combine subcategories

Even though SwiftUI and Combine are both closed source and platform specific frameworks (I hope they'll get eventually open sourced so more platforms will benefit from them just like other platforms benefit form GCD), I think they deserve an own subcategory in the forums.

At least SwiftUI feels like it's home here.

I would love if we could have #related-projects:swiftui, and maybe #related-projects:combine.

Related thread: Pitch: Add iOS, macOS, Linux categories under "Using Swift"

cc @forum_admins, @tkremenek