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")
+    }
+
+}