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/Sources/App/App+build.swift

@@ -65,8 +65,9 @@ private func logger(
 
 /// Builds the application's router.
 ///
-/// Registers the request-logging middleware and the static file middleware that serves
-/// the contents of `staticFilesPath`.
+/// Registers, in order, the request-logging middleware, the not-found middleware that serves
+/// the error page, and the static file middleware that serves the contents of `staticFilesPath`
+/// (falling back to `index.html` for directory requests).
 /// - Parameters:
 ///   - staticFilesPath: the folder, relative to the working directory, the static files are served from.
 ///   - logLevel: the level the request-logging middleware logs at.
@@ -79,9 +80,10 @@ private func router(
 
     router.addMiddleware {
         LogRequestsMiddleware(logLevel)
+        NotFoundMiddleware(staticFilesPath)
         FileMiddleware(
             staticFilesPath,
-            searchForIndexHtml: false
+            searchForIndexHtml: true
         )
     }