Add support for shadow modifier

Closes TokamakUI#324

Sources/TokamakCore/Modifiers/Shadow.swift

@@ -0,0 +1,21 @@
+public struct _ShadowLayout: ViewModifier {
+  public var color: Color
+  public var radius: CGFloat
+  public var x: CGFloat
+  public var y: CGFloat
+
+  public func body(content: Content) -> some View {
+    content
+  }
+}
+
+public extension View {
+  func shadow(
+    color: Color = Color(.sRGBLinear, white: 0, opacity: 0.33),
+    radius: CGFloat,
+    x: CGFloat = 0,
+    y: CGFloat = 0
+  ) -> some View {
+    modifier(_ShadowLayout(color: color, radius: radius, x: x, y: y))
+  }
+}

Sources/TokamakDemo/ShadowDemo.swift

@@ -0,0 +1,8 @@
+import TokamakShim
+
+struct ShadowDemo: View {
+  var body: some View {
+    Color.red.frame(width: 60, height: 60, alignment: .center)
+      .shadow(color: .black, radius: 5, x: 10, y: 10)
+  }
+}

Sources/TokamakDemo/TokamakDemo.swift

@@ -120,6 +120,9 @@ struct TokamakDemoView: View {
             }.padding(20))
             NavItem("GeometryReader", destination: GeometryReaderDemo())
           }
+          Section(header: Text("Modifiers")) {
+            NavItem("Shadow", destination: ShadowDemo())
+          }
           Section(header: Text("Selectors")) {
             NavItem("Picker", destination: PickerDemo())
             NavItem("Slider", destination: SliderDemo())

Sources/TokamakStaticHTML/Modifiers/LayoutModifiers.swift

@@ -101,3 +101,11 @@ extension _PaddingLayout: DOMViewModifier {
       .joined(separator: " ")]
   }
 }
+
+extension _ShadowLayout: DOMViewModifier {
+  public var attributes: [HTMLAttribute: String] {
+    ["style": "box-shadow: \(x)px \(y)px \(radius)px 0px \(color.cssValue(.defaultEnvironment));"]
+  }
+
+  public var isOrderDependent: Bool { true }
+}