Fix eslint/phpcs/phpstan issues

Anjali Mehta → made their first commit to this issue’s fork.

I am working on it.

dieterholvoet → changed the visibility of the branch 1.0.x to hidden.

dieterholvoet → changed the visibility of the branch 3422332-1.0.x to hidden.

Fix the eslint warnings and fix some phpstan errors.

The pipeline is not passing yet.

tom konda → made their first commit to this issue’s fork.

ankitv18 → changed the visibility of the branch 3422332-fix-gitlab-pipeline to hidden.

I fixed all of the ESLINT / PHPUnit tests issues and it's all green now: https://git.drupalcode.org/project/mailchimp_transactional/-/merge_reque...

I had to refactor the MailchimpTransactionalPluginMailTest.php file to make the test to pass, anyways, the tests need some refactoring, I'm not sure why $this->config /code>is <code>NULL in the TransactionMail class when running the PHPUnit tests.

We'll have to drop support for PHP 7.3 if we're adding property types, but I think that's acceptable. Left some comments in the MR.

I fixed all issues mentioned in the comments above, except for the Mail test of course

https://git.drupalcode.org/issue/mailchimp_transactional-3422332/-/jobs/...

1) MailchimpTransactionalPluginMailTest::testMailPluginMailFunction
Tested sending message to multiple recipients.
Failed asserting that false is true.
/builds/issue/mailchimp_transactional-3422332/tests/src/Functional/MailchimpTransactionalPluginMailTest.php:32
FAILURES!
Tests: 8, Assertions: 23, Failures: 1.

I can't think of a solution at the moment, will revisit this later shortly I hope.