Add shadow modifier.

Part of #51.

Sources/Slipstream/Documentation.docc/Guides/SlipstreamForTailwindCSSDevelopers.md

@@ -152,8 +152,8 @@ Tailwind utility | Slipstream modifier
 
 Tailwind utility | Slipstream modifier
 :----------------|:-------------------
-[Box Shadow](https://tailwindcss.com/docs/box-shadow) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/51)
-[Box Shadow Color](https://tailwindcss.com/docs/box-shadow-color) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/51)
+[Box Shadow](https://tailwindcss.com/docs/box-shadow) | ``View/shadow(color:radius:condition:)``
+[Box Shadow Color](https://tailwindcss.com/docs/box-shadow-color) | ``View/shadow(color:radius:condition:)
 [Opacity](https://tailwindcss.com/docs/opacity) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/51)
 [Mix Blend Mode](https://tailwindcss.com/docs/mix-blend-mode) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/51)
 [Background Blend Mode](https://tailwindcss.com/docs/background-blend-mode) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/51)

Sources/Slipstream/Documentation.docc/TailwindCSS/Effects/Effects-Shadow.md

@@ -0,0 +1,9 @@
+# Shadow
+
+Utilities for controlling the box shadow of a view.
+
+## Topics
+
+### Modifiers
+
+- ``View/shadow(color:radius:condition:)``

Sources/Slipstream/Documentation.docc/TailwindCSS/TailwindCSS-Utilities.md

@@ -51,6 +51,10 @@ Slipstream implementations of Tailwind CSS's utility classes.
 - <doc:Borders-Border>
 - <doc:Borders-CornerRadius>
 
+### Effects
+
+- <doc:Effects-Shadow>
+
 ### Filters
 
 - <doc:Filters-BackdropBlur>

Sources/Slipstream/TailwindCSS/Effects/View+shadow.swift

@@ -0,0 +1,38 @@
+extension View {
+  /// Sets the shadow of the view.
+  ///
+  /// - SeeAlso: Tailwind CSS' [box shadow](https://tailwindcss.com/docs/box-shadow) documentation.
+  /// - SeeAlso: Tailwind CSS' [box shadow color](https://tailwindcss.com/docs/box-shadow-color) documentation.
+  @available(iOS 17.0, macOS 14.0, *)
+  public func shadow(
+    color: Color? = nil,
+    radius: Double? = nil,
+    condition: Condition? = nil
+  ) -> some View {
+    var classNames: [String] = []
+    if let color {
+      classNames.append("shadow-\(color.toTailwindColorClass())")
+    }
+    if let radius {
+      classNames.append("shadow" + Self.radiusToTailwindShadowClass(radius: radius))
+    }
+    return modifier(TailwindClassModifier(add: Set(classNames), condition: condition))
+  }
+
+  /// Maps a shadow radius to the closest Tailwind CSS shadow class.
+  ///
+  /// - Parameter radius: The radius in points to be mapped.
+  /// - Returns: The Tailwind CSS shadow class.
+  private static func radiusToTailwindShadowClass(radius: Double) -> String {
+    let mapping: [(name: String, radius: Double)] = [
+      ("-sm", 2),
+      ("", 3),
+      ("-md", 6),
+      ("-lg", 15),
+      ("-xl", 25),
+      ("-2xl", 50),
+    ]
+    let closestClass = mapping.min { abs($0.radius - radius) < abs($1.radius - radius) }
+    return closestClass?.name ?? "0"
+  }
+}