Add flag in site create to support WP_ENVIRONMENT_TYPE

[mrrobot47]

Add flag, say --env=<value> in site create, which will add that as below in wp-config.php:

define( 'WP_ENVIRONMENT_TYPE', 'value' );

For the initial release of this feature, we will restrict the values that this flag can take as input to:
production, staging and development which are the default 3 envs defined in https://make.wordpress.org/core/2020/07/24/new-wp_get_environment_type-function-in-wordpress-5-5/