Add image gallery block

CHDataManagement/Generator/Blocks/GalleryBlock.swift

@@ -0,0 +1,46 @@
+
+struct GalleryBlock: BlockLineProcessor {
+
+    static let blockId: ContentBlock = .gallery
+
+    let content: Content
+
+    let results: PageGenerationResults
+
+    let language: ContentLanguage
+
+    init(content: Content, results: PageGenerationResults, language: ContentLanguage) {
+        self.content = content
+        self.results = results
+        self.language = language
+    }
+
+    private var imageWidth: Int {
+        content.settings.pages.contentWidth
+    }
+
+    func process(_ lines: [String], markdown: Substring) -> String {
+        var images = [FileResource]()
+
+        for line in lines {
+            let imageId = line.trimmed
+            guard !imageId.isEmpty else { continue }
+            guard let image = content.image(imageId) else {
+                results.missing(file: imageId, source: "Route block")
+                continue
+            }
+            images.append(image)
+        }
+        guard let firstImage = images.first else { return "" }
+
+        let imageSets = images.map {
+            $0.imageSet(width: imageWidth, height: imageWidth, language: language)
+        }
+        imageSets.forEach(results.require)
+        let id = firstImage.id.replacingOccurrences(of: ".", with: "-")
+        let gallery = ImageGallery(id: id, images: imageSets, standalone: true)
+        results.require(footer: gallery.standaloneFooter)
+        results.require(headers: .swiperJs, .swiperCss)
+        return gallery.content
+    }
+}