Add pause button.

[AayushiJapsare15]

What does this PR do?

Pause button is added which appears when game is started and hides when stopped.

Fixes #88

Type of change

• Chore (refactoring code, technical debt, workflow improvements)
• New feature (non-breaking change which adds functionality)

How should this be tested?

• start game and press pause button.

Mandatory Tasks

• Make sure you have self-reviewed the code. A decent size PR without self-review might be rejected.

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[AayushiJapsare15]

@YadavAkhileshh Please add hacktoberfest-accpeted label, as it is not accepting this PR and others as well.