Add display model

Sources/SwiftUIMath/Internal/Display/DisplayAccent.swift

@@ -0,0 +1,24 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayAccent: DisplayNode {
+    var accentee: DisplayList?
+    var accent: DisplayGlyph?
+
+    init(accent: DisplayGlyph?, accentee: DisplayList?, range: NSRange) {
+      self.accent = accent
+      self.accentee = accentee
+      super.init()
+      self.range = range
+    }
+
+    override var position: CGPoint {
+      didSet { updateAccenteePosition() }
+    }
+
+    private func updateAccenteePosition() {
+      accentee?.position = CGPoint(x: position.x, y: position.y)
+    }
+  }
+}