feat: add string_tools example with basic string utilities

[NA-V10]

Summary

This PR adds a new example module string_tools in the examples/ directory.
It demonstrates how to expose multiple string utility functions as a Fire CLI.

Details

• Added folder: examples/string_tools/
• Added functions: reverse, uppercase, count_words
• Included simple test file following the pattern of existing examples
• Structured similarly to cipher/ and diff/

Checklist

• Code runs successfully
• Example follows existing directory pattern
• Includes test file
• Does not modify existing Fire functionality

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