fix: add string type to gt/lt/gte/lte type definitions

[damieng]

🔗 Linked issue

#437

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The $gte, $gt, $lt, $lte operators are defined in the Typescript as only taking a number. This is in fact incorrect as they map to the standard JavaScript >=, >, <, <= operators which can handle any more types and as the test coverage shows they work perfect with strings.

This fix is needed as anyone trying to use these operators with a string - such as in a common case as filtering by alpha ranges or even a sort-formatted date string - will receive Typescript errors leading them to think this is not the way to do it.

This adds the string type to those operators, adds test coverage to prove they work and importantly adds an example to the filtering docs showing how to use the string filtering to sort by a date which is a common situation and one the linked ticket went into.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[farnabaz]

Thanks