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/Sources/Methods/Localize.swift

@@ -0,0 +1,45 @@
+import Foundation
+
+/// A reusable, bundle-bound localizer that resolves String Catalog entries for an explicit locale.
+///
+/// A server has no single "current" locale, so each lookup must name the locale to use. An instance
+/// is bound to the bundle whose catalog holds the strings, then invoked like a function to resolve a
+/// key in a chosen locale.
+public struct Localize: Sendable {
+
+    // MARK: Properties
+
+    /// The bundle whose compiled String Catalog the keys are resolved against.
+    private let bundle: Bundle
+
+    // MARK: Initializers
+
+    /// Creates a localizer backed by the given bundle.
+    /// - Parameter bundle: the bundle whose String Catalog contains the keys to resolve.
+    public init(
+        bundle: Bundle
+    ) {
+        self.bundle = bundle
+    }
+
+    // MARK: Methods
+
+    /// Resolves a catalog key in the given locale.
+    ///
+    /// Invoked by calling the instance directly, for example `localize("index.title", locale: locale)`.
+    /// - Parameters:
+    ///   - key: the String Catalog key to look up.
+    ///   - locale: the locale to resolve the key in.
+    /// - Returns: the localized string for the locale, or the key itself when the bundle's catalog has no entry for it.
+    public func callAsFunction(
+        _ key: String.LocalizationValue,
+        locale: Locale
+    ) -> String {
+        .init(localized: .init(
+            key,
+            locale: locale,
+            bundle: .atURL(bundle.bundleURL)
+        ))
+    }
+
+}