Given a List of strings, return a Map where keys are the first letters (uppercase) and values are Lists of strings starting with that letter. Sort the strings within each list alphabetically.
Examples:
Input: words = ['apple', 'banana', 'apricot', 'blueberry', 'cherry']
Output: {'A' => ['apple', 'apricot'], 'B' => ['banana', 'blueberry'], 'C' => ['cherry']}
Explanation: Strings grouped by uppercase first letter and sorted
Input: words = ['Hello', 'world', 'Hi', 'Welcome']
Output: {'H' => ['Hello', 'Hi'], 'W' => ['Welcome', 'world']}
Explanation: First letters are uppercase keys, case-insensitive grouping
Input: words = []
Output: {}
Explanation: Empty input returns empty map
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