This PR adds a new R script unique.letters.count.R under the string_manipulation directory.
The program prompts the user for a string, cleans it by removing non-letter characters, converts it to lowercase, and counts the occurrences of each unique letter.

Additionally, it includes minor refactoring for clarity and efficiency as per review suggestions.

1. Prompts the user to enter a string.
2. Removes all non-letter characters (e.g., numbers, punctuation, symbols).
3. Converts text to lowercase for case-insensitive counting.
4. Splits the cleaned string into individual letters.
5. Identifies and counts occurrences of each unique letter.
6. Displays the result in a simple, readable format.
7. Uses dot notation (clean.string, unique.letters, etc.) for consistency with R naming conventions.