Proposal: Add New Exercise - "Isogram Checker"

[003-kalki]

Proposal: Add New Exercise - "Isogram Checker"

Description:

An isogram is a word or phrase where no letter repeats. This exercise will challenge users to implement a function that checks whether a given string is an isogram.

Why Add This?

• Helps learners practice string manipulation.
• Introduces case insensitivity handling in comparisons.
• Useful for interview preparation and reinforcing fundamental logic skills.

###Example Test case:
isIsogram("playground"); // ➞ true
isIsogram("banana"); // ➞ false
isIsogram("Dermatoglyphics"); // ➞ true
isIsogram("hello world"); // ➞ false
isIsogram("Isogram"); // ➞ true