File not found middleware for the Website service (#4)

This PR contains the work done to add a `NotFoundMiddleware` middleware to the Website service so requests matching neither a route nor a static file return a custom 404.html page with a 404 Not Found status. Also enables index.html fallback so the landing page is served at the site root.

To provider further details about the work done:
* NotFoundMiddleware — intercepts the `.notFound` error from the  `FileMiddleware` middleware and serves the preloaded error page with the correct content type; all other errors propagate. Falls back to a minimal body if the file is missing.
* Router — wires the `NotFoundMiddleware` middleware ahead of the `FileMiddleware` and enables searchForIndexHtml.
* Package — adds the **Hummingbird** product to the Library target.
* Tooling — sets a custom working directory in the Xcode scheme; removes the unused `pkg-deps` target from the `Makefile` file.

Reviewed-on: rock-n-code/loud-amsterdam#4
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Package.swift

@@ -55,6 +55,10 @@ let package = Package(
                     name: "Configuration",
                     package: "swift-configuration"
                 ),
+                .product(
+                    name: "Hummingbird",
+                    package: "hummingbird"
+                ),
             ],
             path: "Sources/Library"
         ),