Modified from_str() to perform case-insensitive comparison for all level names.
This allows environment variables and SPDLOG_LEVEL_NAMES to use uppercase or
mixed case level names (e.g., DEBUG, INFO, Warning) while maintaining full
backward compatibility with existing lowercase names.
SPDLOG_LEVEL is currently supported to control log levels via
load_env_levels.
This patch adds support for other environment variable names, such as
MYAPP_LEVEL, for load_env_levels.