File-based response support & static server

- Clean up UniversalPath implementation
    - Use Readable/Writable types correctly for stream methods
    - Add .list() methods for getting child files

- Make Response body specify explicit types and support
  writing Readable streams to the body

- Create a static file server that supports directory listing

src/http/HTTPError.ts

@@ -10,8 +10,9 @@ export class HTTPError extends ErrorWithContext {
     constructor(
         public readonly status: HTTPStatus = 500,
         public readonly message: string = '',
+        context?: {[key: string]: any},
     ) {
-        super('HTTP ERROR')
+        super('HTTP ERROR', context)
         this.message = message || HTTPMessage[status]
     }
 }