Add a random string generator for javascript

[kruimol]

Description

Adds a snipped that generate a random string with a certain length.

Type of Change

• [ x ] ✨ New snippet
• 🛠 Improvement to an existing snippet
• 🐞 Bug fix
• 📖 Documentation update
• 🔧 Other (please describe):

Checklist

• [ x ] I have tested my code and verified it works as expected.
• [ x ] My code follows the style and contribution guidelines of this project.
• [ x ] Comments are added where necessary for clarity.
• [ x ] Documentation has been updated (if applicable).
• [ x ] There are no new warnings or errors from my changes.

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