Fix PHP 8: declare required args before optional args

[uday-kokitkar]

Fixes PHP deprecation in v8.0.0.
PHP Deprecated: Required parameter $submit_text follows optional parameter $list in yikes-inc-easy-mailchimp-extender/public/helpers.php on line 14

More about function arguments: https://www.php.net/manual/en/functions.arguments.php

[uday-kokitkar]

Re-opening the PR. I found that the function yksemeProcessSnippet is not being used by this plugin anywhere. I can see in the doc block that it was written to support legacy PHP snippets.
So wherever those snippets are present, and calls to this function are happening, change in the arguments order is necessary.

I have set the default value of $submit_text to null. PHP isset returns false to null values. No need to change the arguments sequence.