feat: mount should support dynamic routes

[felangel]

As a developer, I want to be able to mount routes to a dynamic prefix so that I can have a consistent way to structure my router and apply middleware.

import 'dart:io';

import 'package:shelf/shelf.dart';
import 'package:shelf/shelf_io.dart';
import 'package:shelf_router/shelf_router.dart';

final _router = Router()..mount('/<echo>', (r) => _echoHandler()(r));

Handler _echoHandler() {
  var pipeline = const Pipeline();
  final router = Router()..get('/', (_) => Response.ok('hi'));
  return pipeline.addHandler(router);
}

void main(List<String> args) async {
  final ip = InternetAddress.anyIPv4;
  final handler = Pipeline().addMiddleware(logRequests()).addHandler(_router);
  final port = int.parse(Platform.environment['PORT'] ?? '8080');
  final server = await serve(handler, ip, port);
  print('Server listening on port ${server.port}');
}

I expect to be able to run the above server and make a GET request to http://localhost:8080/hello and get a 200 response with hi.

Instead, running the above code and making a GET request to http://localhost:8080/hello results in a 500 internal server error

ERROR - 2022-07-04 15:16:20.694525
GET /hello
Error thrown by handler.
RangeError (start): Invalid value: Not in inclusive range 0..5: 8
package:shelf/src/request.dart 258:20              Request.change
package:shelf_router/src/router.dart 163:32        Router.mount.<fn>
package:shelf_router/src/router_entry.dart 105:30  RouterEntry.invoke.<fn>
package:shelf_router/src/router_entry.dart 103:30  RouterEntry.invoke.<fn>
package:shelf_router/src/router_entry.dart 111:7   RouterEntry.invoke
package:shelf_router/src/router.dart 183:38        Router.call

Related to https://github.com/google/dart-neats/issues/40

[davidmartos96]

@felangel I made a PR for this functionality in case you want to give it a try. #288

[felangel]

@felangel I made a PR for this functionality in case you want to give it a try. #288

Thanks I'll take a look shortly 💯

EDIT

I played with the branch and the changes appear to behavior as expected. I'll plan to test the changes more thoroughly in the coming days.