Add documentation for JavaScript String trimStart() method

[Tia-ani]

This PR adds a new entry for the trimStart() method under JavaScript strings concepts.

The documentation includes:

• Method description and behavior
• Syntax and return value details
• Practical examples showing whitespace removal
• Interactive codebyte demonstrating various use cases

File added:
content/javascript/concepts/strings/terms/trimStart/trimStart.md

• Solved issue [Term Entry] JavaScript Strings: trimStart() #7686

[CLAassistant]

All committers have signed the CLA.

[mamtawardhani]

Looks good for a second review! 🚀

[avdhoottt]

LGTM!!

[github-actions]

👋 @Tia-ani
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