Add display model

Sources/SwiftUIMath/Internal/Display/DisplayNode.swift

@@ -0,0 +1,17 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  class DisplayNode {
+    var ascent: CGFloat = 0
+    var descent: CGFloat = 0
+    var width: CGFloat = 0
+    var position: CGPoint = .zero
+    var range: NSRange = NSRange(location: 0, length: 0)
+    var hasScript: Bool = false
+
+    func bounds() -> CGRect {
+      CGRect(x: position.x, y: position.y - descent, width: width, height: ascent + descent)
+    }
+  }
+}