Added the ImageWidth enumeration to the Website library target.

Services/Website/Sources/Library/Internal/Enumerations/ImageWidth.swift

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+/// A rendition an image asset is available at, from the narrowest up to the full-size file.
+///
+/// ``StaticFile`` names one file per rendition, and a page's `srcset` offers them all so the browser picks by rendered width.
+/// The declaration order is the `srcset` order: narrowest first.
+enum ImageWidth: CaseIterable {
+    /// The small rendition, 480 pixels wide.
+    case small
+    /// The medium rendition, 800 pixels wide.
+    case medium
+    /// The large rendition: the full-size file, 1200 pixels wide.
+    case large
+}
+
+// MARK: - Extensions
+
+extension ImageWidth {
+
+    // MARK: Computed
+
+    /// The rendition's width in pixels: what the file is resampled to, and what its `srcset` descriptor states.
+    var width: Int {
+        switch self {
+        case .small: 480
+        case .medium: 800
+        case .large: 1200
+        }
+    }
+
+}