Localization support for the Website service (#10)

This PR contains the work done to introduce server-side localization support to the **Website** service so the landing and error pages are served in the visitor's negotiated language, backed by a new reusable Localization package.

To provide further details about the work:

* Created the _Localization_ package — a bundle-bound `Localize` method and a `LanguageList` type.
* Language negotiation — `NegotiateLanguage` method picks the best supported language from the request's _Accept-Language_ header (falling back to the default); the `LocalizationMiddleware` middleware resolves it per request and stores it on a new `LocalizedRequestContext` / `WebsiteRequestContext` context.
* Localized responses — `LocalizedHTMLCollectionResponse` pre-renders each page once per language and caches the bytes (with `Content-Language` + `Vary: Accept-Language`), reused by the  `RootController` controller and `NotFoundMiddleware` middleware.
* The `CachedHTMLResponse` response gained custom-header support.
* Localized pages — the `IndexPage` and `ErrorPage` pages now resolve their strings via `Localize` method;
* Added `Localizable.xcstrings` catalogs.
* Wired the `LocalizationMiddleware` middlewaer into the router.

Reviewed-on: rock-n-code/loud-amsterdam#10
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Packages/Localization/Tests/Cases/Methods/LocalizeTests.swift

@@ -0,0 +1,45 @@
+import Foundation
+import Testing
+
+@testable import Localization
+
+@Suite("Localize method")
+struct LocalizeTests {
+    
+    // MARK: Constants
+    
+    private let localize = Localize(bundle: .module)
+
+    // MARK: Functional tests
+
+    @Test
+    func `resolves a key in the default locale`() {
+        let text = localize(
+            "test.greeting",
+            locale: Locale(identifier: "en")
+        )
+
+        #expect(text == "Hello")
+    }
+
+    @Test
+    func `resolves a key in another locale`() {
+        let text = localize(
+            "test.greeting",
+            locale: Locale(identifier: "de"),
+        )
+
+        #expect(text == "Hallo")
+    }
+
+    @Test
+    func `falls back to the key for an unknown entry`() {
+        let text = localize(
+            "unknown.key",
+            locale: Locale(identifier: "en"),
+        )
+
+        #expect(text == "unknown.key")
+    }
+
+}

Packages/Localization/Tests/Cases/Types/LanguageListTests.swift

@@ -0,0 +1,65 @@
+import Foundation
+import Testing
+
+@testable import Localization
+
+@Suite("LanguageList type")
+struct LanguageListTests {
+
+    // MARK: Properties tests
+
+    @Suite("default")
+    struct Default {
+        @Test
+        func `defaults to english`() {
+            let list = LanguageList(
+                bundle: .module
+            )
+
+            #expect(list.default == "en")
+        }
+
+        @Test
+        func `uses the provided default language`() {
+            let list = LanguageList(
+                bundle: .module,
+                default: "de"
+            )
+
+            #expect(list.default == "de")
+        }
+    }
+
+    @Suite("all")
+    struct All {
+        @Test
+        func `lists the catalog languages`() {
+            let list = LanguageList(
+                bundle: .module
+            )
+
+            #expect(list.all.contains("en"))
+            #expect(list.all.contains("de"))
+        }
+
+        @Test
+        func `excludes the base localization`() {
+            let list = LanguageList(
+                bundle: .module
+            )
+
+            #expect(!list.all.contains("Base"))
+        }
+
+        @Test
+        func `lists each language only once`() {
+            let list = LanguageList(
+                bundle: .module
+            )
+            let all = list.all
+
+            #expect(all.count == Set(all).count)
+        }
+    }
+
+}