Add new token modifier: Lower case first

This module provides several plugins to alter token values, such as lowercase, uppercase, and upper-case-first. However, there is currently no modifier to transform only the first character of a token to lowercase.

Proposed solution

• Add a new plugin named lower-case-first.
• The implementation can be directly based on the existing upper-case-first plugin for consistency.
• Include a unit test following the same structure and assertions used in the upper-case-first test.
• Update the readme (+ fix upper-case-first example)

Example