Add Canonical. (#185)

Part of #25.

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

@@ -29,7 +29,7 @@ provided below is an organized table of W3C HTML tags and their equivalent Slips
 [`<base>`](https://html.spec.whatwg.org/multipage/semantics.html#the-base-element) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25)
 [`<link rel="alternate">`](https://html.spec.whatwg.org/multipage/links.html#rel-alternate) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25)
 [`<link rel="alternate">`](https://html.spec.whatwg.org/multipage/links.html#rel-alternate) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
-[`<link rel="canonical">`](https://html.spec.whatwg.org/multipage/links.html#link-type-canonical) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
+[`<link rel="canonical">`](https://html.spec.whatwg.org/multipage/links.html#link-type-canonical) | ``Canonical`` 
 [`<link rel="author">`](https://html.spec.whatwg.org/multipage/links.html#link-type-author) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
 [`<link rel="bookmark">`](https://html.spec.whatwg.org/multipage/links.html#link-type-bookmark) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 
 [`<link rel="dns-prefetch">`](https://html.spec.whatwg.org/multipage/links.html#link-type-dns-prefetch) | [Not implemented yet](https://github.com/jverkoey/slipstream/issues/25) 

Sources/Slipstream/Documentation.docc/W3C/W3CViews.md

@@ -14,6 +14,7 @@ The complete W3C HTML elements standard can be found [here](https://html.spec.wh
 
 - ``Head``
 - ``Title``
+- ``Canonical``
 - ``Icon``
 - ``Preload``
 - ``Preconnect``

Sources/Slipstream/W3C/Elements/DocumentMetadata/Canonical.swift

@@ -0,0 +1,39 @@
+import Foundation
+
+import SwiftSoup
+
+/// A view that indicates that a URL is the preferred URL for the current document.
+///
+/// ```swift
+/// struct MySiteMetadata: View {
+///   var body: some View {
+///     Head {
+///       Canonical(URL(string: "https://mysite.com"))
+///     }
+///   }
+/// }
+/// ```
+///
+/// - SeeAlso: W3C [rel="canonical"](https://html.spec.whatwg.org/multipage/links.html#link-type-canonical) specification.
+@available(iOS 17.0, macOS 14.0, *)
+public struct Canonical: View {
+  /// Creates a Canonical view.
+  ///
+  /// - Parameters:
+  ///   - url: The preferred URL for this document.
+  public init(_ url: URL?) {
+    self.url = url
+  }
+
+  @_documentation(visibility: private)
+  public func render(_ container: Element, environment: EnvironmentValues) throws {
+    guard let url else {
+      return
+    }
+    let element = try container.appendElement("link")
+    try element.attr("rel", "canonical")
+    try element.attr("href", url.absoluteString)
+  }
+
+  private let url: URL?
+}

Tests/SlipstreamTests/W3C/CanonicalTests.swift

@@ -0,0 +1,14 @@
+import Foundation
+import Testing
+
+import Slipstream
+
+struct CanonicalTests {
+  @Test func nilURL() throws {
+    try #expect(renderHTML(Canonical(nil)) == "")
+  }
+
+  @Test func validURL() throws {
+    try #expect(renderHTML(Canonical(URL(string: "https://mysite.com"))) == #"<link rel="canonical" href="https://mysite.com" />"#)
+  }
+}