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>

Services/Website/Sources/Library/Public/Middlewares/LocalizationMiddleware.swift

@@ -0,0 +1,56 @@
+import HTTPTypes
+import Hummingbird
+import Localization
+
+/// Resolves the visitor's preferred language and records it on the request context.
+///
+/// Placed ahead of the localized responders in the middleware chain, it reads the request's
+/// `Accept-Language` header, negotiates the best supported match (falling back to the default
+/// language), and stores it on the context's ``LocalizedRequestContext/language``.
+///
+/// The request is otherwise passed through untouched — the URL and routing are not affected — so
+/// each page is served at its existing path and varies its content by header.
+public struct LocalizationMiddleware<Context: LocalizedRequestContext> {
+
+    // MARK: Properties
+
+    /// Negotiates the request's language from its `Accept-Language` header.
+    private let negotiate: NegotiateLanguage
+
+    // MARK: Initializers
+
+    /// Creates a localization middleware.
+    public init() {
+        self.negotiate = .init()
+    }
+
+}
+
+// MARK: - RouterMiddleware
+
+extension LocalizationMiddleware: RouterMiddleware {
+
+    // MARK: Functions
+
+    /// Negotiates the request's language and records it on the context before passing it down.
+    /// - Parameters:
+    ///   - request: the incoming request.
+    ///   - context: the context the request is resolved against.
+    ///   - next: the next responder in the middleware chain.
+    /// - Returns: the downstream response.
+    /// - Throws: any error thrown downstream.
+    public func handle(
+        _ request: Request,
+        context: Context,
+        next: (Request, Context) async throws -> Response
+    ) async throws -> Response {
+        var context = context
+
+        context.language = negotiate(
+            forAcceptLanguage: request.headers[.acceptLanguage]
+        )
+
+        return try await next(request, context)
+    }
+
+}