feat: Add delete all mindful sessions method

[ahoseinian]

Description

in our app we needed a functionality to remove all the mindful session that we have stored for the user at once. since that functionality was not provided by the library. I decided to add the function deleteAllMindfulSessions and make the PR for it.

Fixes # ( no issue created yet )

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] I have checked my code and corrected any misspellings