Re-arrange source code and tests

Sources/SwiftUIMath/Internal/Syntax/Atoms/Style.swift

@@ -0,0 +1,41 @@
+import Foundation
+
+extension Math {
+  final class Style: Atom {
+    enum Level: Int {
+      case display
+      case text
+      case script
+      case scriptOfScript
+
+      var isScript: Bool {
+        switch self {
+        case .script, .scriptOfScript:
+          return true
+        default:
+          return false
+        }
+      }
+
+      var isNotScript: Bool {
+        !isScript
+      }
+
+      func next() -> Level {
+        Level(rawValue: rawValue + 1) ?? .display
+      }
+    }
+
+    var level: Level
+
+    init(_ style: Style) {
+      self.level = style.level
+      super.init(style)
+    }
+
+    init(level: Level = .display) {
+      self.level = level
+      super.init(type: .style)
+    }
+  }
+}