Improve support for environment types and fix description of plugin

[mcaskill]

Fixed the description of the plugin in the README to indicate that it relies on the value of a DISALLOW_INDEXING constant and not WP_ENV. The latter constant only being used as a reference in an dashboard notice.

Also added support for WP_ENVIRONMENT_TYPE and support for omitting WP_ENV. Proposed behaviour:

1. Resolve the environment type:
   1. Use the value of the WP_ENV constant if defined and palpable.
   2. Otherwise, use the value of the wp_get_environment_type() function if defined (introduced in WordPress 5.0.0).
   3. Otherwise, use NULL.
2. Filter the environment type with the following hook:
   • roots/bedrock/disallow_indexing_environment_type
3. Display the notice:
   1. If the environment type is palpable, display the notice with the filtered environment type.
   2. Otherwise, display a shorter notice that does not reference the environment type.

[retlehs]

Thank you for the PR! My apologies that this slipped through the cracks.

Thanks for the README clarification about DISALLOW_INDEXING being the key constant. The new filter hook is also a good addition.

I'm not a fan of if / else's, what about something like this?

add_action('admin_notices', function () {
    $env = defined('WP_ENV') && WP_ENV ? WP_ENV : null;
    
    if (!$env && function_exists('wp_get_environment_type')) {
        $env = wp_get_environment_type();
    }

    $env = apply_filters('roots/bedrock/disallow_indexing_environment_type', $env);
    
    if (!$env) {
        echo "<div class='notice notice-warning'><p><strong>Bedrock:</strong> " . 
             __('Search engine indexing has been discouraged.', 'roots') . 
             "</p></div>";
        return;
    }

    printf(
        "<div class='notice notice-warning'><p>%s</p></div>",
        sprintf(
            __('%1$s Search engine indexing has been discouraged because the current environment is %2$s.', 'roots'),
            '<strong>Bedrock:</strong>',
            '<code>' . $env . '</code>'
        )
    );
});

Also noting that Bedrock added support for WP_ENVIRONMENT_TYPE a bit after this PR: roots/bedrock#668

[mcaskill]

@retlehs Thanks for your suggestion. I've included it in my changeset.