Re-arrange source code and tests

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

@@ -0,0 +1,27 @@
+import Foundation
+
+extension Math {
+  final class Underline: Atom {
+    var innerList: AtomList?
+
+    override var finalized: Math.Atom {
+      let finalized = super.finalized
+
+      if let underline = finalized as? Underline {
+        underline.innerList = underline.innerList?.finalized
+      }
+
+      return finalized
+    }
+
+    init(_ underline: Underline) {
+      self.innerList = underline.innerList.map { AtomList($0) }
+      super.init(underline)
+    }
+
+    init(innerList: AtomList? = nil) {
+      self.innerList = innerList
+      super.init(type: .underline)
+    }
+  }
+}