Enhance Gameplay by Adding Difficulty Level

[Shweta-1902]

What does this PR do?

This pull request adds a difficulty level selector to the game, enhancing gameplay by allowing users to choose between Easy, Medium, and Hard difficulty levels. Changes have been made to the HTML, CSS, and JavaScript files to improve the game's visuals and adjust the speed and number of aliens falling based on the selected difficulty level.

Fixes #152

##scrnshots:

Type of change

• New feature (non-breaking change which adds functionality)

How should this be tested?

• Start the game and observe the new difficulty level selector.
• Test each difficulty level to ensure the speed and number of aliens adjust accordingly.

Mandatory Tasks

• Self-reviewed the code.

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[github-actions]

🎉 Thank you @Shweta-1902 for your contribution! Your pull request has been submitted successfully. A maintainer will review it as soon as possible. We appreciate your support in making this project better

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