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/Package.swift

@@ -0,0 +1,37 @@
+// swift-tools-version:6.3
+
+import PackageDescription
+
+let package = Package(
+    name: "Localization",
+    defaultLocalization: "en",
+    platforms: [
+        .macOS(.v15),
+        .iOS(.v18),
+        .tvOS(.v18),
+    ],
+    products: [
+        .library(
+            name: "Localization",
+            targets: [
+                "Localization"
+            ]
+        )
+    ],
+    targets: [
+        .target(
+            name: "Localization",
+            path: "Sources",
+        ),
+        .testTarget(
+            name: "LocalizationTests",
+            dependencies: [
+                .byName(name: "Localization")
+            ],
+            path: "Tests",
+            resources: [
+                .process("Catalogs/Localizable.xcstrings")
+            ]
+        ),
+    ]
+)