Adding Flutter Tutorials

[RivaanRanawat]

Added Flutter Tutorials in Dart Subsection

Description

Added Flutter Full Stack Projects and UI Tutorials to the list. Created New List with Dart as the main category and Flutter as sub category.

Motivation and Context

Fixes issue #282 - https://github.com/practical-tutorials/project-based-learning/issues/282

How Has This Been Tested?

All the tutorials in this project are up-to-date.

Types of changes

• [x] Content Update (change which fixes an issue or updates an already existing submission)
• [ ] New Article (change which adds functionality)
• [x] Documentation change

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have made checks to ensure URLs and other resources are valid