Add ext-mbstring to list of suggested dependencies

[swissspidy]

IterableCodeExtractor uses mb_ereg.

Fixes #138

[schlessera]

I cannot merge this change, as it would effectively force this requirement for the entire bundled WP-CLI.

Instead of adding a hard requirement to Composer, it should be a suggested optional dependency, and the command should have a before_invoke action that checks for the presence of the extension and exits with a WP_CLI::error() if it is not present.

[swissspidy]

Instead of adding a hard requirement to Composer, it should be a suggested optional dependency, and the command should have a before_invoke action that checks for the presence of the extension and exits with a WP_CLI::error() if it is not present.

Makes sense. Just made the necessary changes 👍