[Term Entry] Dart Queue: .addFirst()

[Utkarsh-Singhal-26]

Description

This PR adds a new term entry for the .addFirst() method in Dart's Queue concept.

Changes made:

• Created a new file: docs/content/dart/concepts/queue/terms/addFirst/addFirst.md
• Added an introduction explaining the .addFirst() method
• Included a Syntax section with clear parameter description
• Provided an Example section demonstrating practical usage of the method with multiple .addFirst() calls
• Showed how elements are inserted at the beginning of the queue

Issue Solved

Closes #8142

Type of Change

• Adding a new entry

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

[mamtawardhani]

The PR looks good for a second review! 🚀