fix: respect NO_COLOR environment variable in dev format

[luisangelrod]

Summary

The dev format currently emits ANSI color escape codes unconditionally. This change follows the NO_COLOR convention: color is disabled when NO_COLOR is present and non-empty, while an empty value preserves the existing behavior.

Changes

index.js

• Check whether NO_COLOR has a non-empty value before compiling the dev format.
• Cache a plain format without ANSI escape codes when color is disabled.
• Preserve the current colored output when NO_COLOR is absent or empty.

test/morgan.js

• Cover color-free 2xx and 5xx output with NO_COLOR=1.
• Cover the convention's empty-value behavior.
• Isolate the dev-format tests from an inherited NO_COLOR shell value and restore the original environment after each test.

Testing

• npm test — 91 passing
• npm run lint
• git diff --check

Closes #302