Added support for RouteGenerator

[infiniteoverflow]

Description

This pull request adds support for RouteGenerator by which we can use named routes to navigate to other screens.

Changes I have made :

• Added a new class in lib/services/route_generator.dart. This contains the backend logic of RouteGenerators.
• Added an error screen in lib/screens/error_screen.dart to show when wrong named routes are used to access some screens.
• Updated the navigation code to ProfilePage screen.
• Updated the navigation code to Pathway screen.

Related Issues

Fixes issue #49

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [x] I read the [Contributor Guide].
• [x] I updated/added relevant comments in code.
• [x] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• [x] All existing and new tests are passing.
• [x] The analyzer (flutter analyze) does not report any problems on my PR.
• [x] I am willing to follow-up on review comments in a timely manner.

[adityathakurxd]

Thank you @infiniteoverflow! Looks great.