Adds default tab stops (#362)

* Adds default tab stops

* Fixes SwiftLint warning

Author: simonbs

Sources/Runestone/Library/DefaultStringAttributes.swift

@@ -9,7 +9,9 @@ struct DefaultStringAttributes {
 
     func apply(to attributedString: NSMutableAttributedString) {
         let paragraphStyle = NSMutableParagraphStyle()
-        paragraphStyle.tabStops = []
+        paragraphStyle.tabStops = (0 ..< 20).map { index in
+            NSTextTab(textAlignment: .natural, location: CGFloat(index) * tabWidth)
+        }
         paragraphStyle.defaultTabInterval = tabWidth
         let range = NSRange(location: 0, length: attributedString.length)
         let attributes: [NSAttributedString.Key: Any] = [