Add display model

Sources/SwiftUIMath/Internal/Display/DisplayTextRun.swift

@@ -0,0 +1,19 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayTextRun: DisplayNode {
+    var text: String
+    var font: Math.Font
+    var atoms: [Math.Atom]
+
+    init(text: String, font: Math.Font, position: CGPoint = .zero, range: NSRange, atoms: [Math.Atom]) {
+      self.text = text
+      self.font = font
+      self.atoms = atoms
+      super.init()
+      self.position = position
+      self.range = range
+    }
+  }
+}