Add --no-color flag to disable ANSI color output

[Chemaclass]

📚 Description

Add support for disabling colored output in bashunit, following the NO_COLOR standard. This allows users to run tests in environments where ANSI escape codes are problematic, such as:

• CI/CD pipelines that log output to files
• Terminals that don't support colors
• When piping output to other tools
• Accessibility tools that work better with plain text

The feature can be enabled via:

1. Command-line flag: --no-color
2. Environment variable: NO_COLOR=1 (following the standard)

Example usage

# Using the flag
./bashunit tests/ --no-color

# Using the environment variable (NO_COLOR standard)
NO_COLOR=1 ./bashunit tests/

# Useful for logging to files without escape sequences
./bashunit tests/ --no-color > test_results.txt

🔖 Changes

• Add --no-color command-line option to disable terminal colors
• Add support for the NO_COLOR environment variable (https://no-color.org standard)
• Add bashunit::env::is_no_color_enabled helper function
• Add BASHUNIT_NO_COLOR environment variable for configuration
• Update src/colors.sh to conditionally set color variables to empty strings when no-color mode is enabled
• Update help text in both test and benchmark modes to document the new option
• Add acceptance tests to verify both the flag and env var disable colors correctly
• Fix existing tests to propagate no-color setting when running nested bashunit calls

🖼️ Demo

✅ To-do list

• I updated the CHANGELOG.md to reflect the new feature or fix
• I updated the documentation to reflect the changes