Print runtime warning in previews. (pointfreeco#135)

* Print runtime warning in previews.

* Use legacy previews

Author: mbrandonw

Examples/Examples/ExamplesApp.swift

@@ -26,3 +26,10 @@ class ItemsModel {
     return lastItem
   }
 }
+
+struct Previews: PreviewProvider {
+  static var previews: some View {
+    let model = ItemsModel()
+    Text(model.lastItem?.description ?? "No last item")
+  }
+}

Sources/IssueReporting/IssueReporters/RuntimeWarningReporter.swift

@@ -66,6 +66,11 @@ public struct _RuntimeWarningReporter: IssueReporter {
     column: UInt
   ) {
     #if canImport(os)
+    guard ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] != "1"
+    else {
+      print("🟣 \(fileID):\(line): \(message() ?? "")")
+      return
+    }
       let moduleName = String(
         Substring("\(fileID)".utf8.prefix(while: { $0 != UTF8.CodeUnit(ascii: "/") }))
       )