Add/community files

[jeffpaul]

This PR:

• adds a code of conduct file
• adds a contributing file
• adds a security policy file
• adds issue templates
• adds PR template
• adds dependabot config file
• composer.json and package.json base files

Closes #13.

[dkotter]

Wondering if we want to set this to 7.4. I know we have a separate task to ensure PHP 8 compat but wondering if there's any reason to start this at such a low number

[jeffpaul]

I set 5.6 in

wordpress/mailchimp.php

Line 8 in afd2d98

* Requires PHP: 5.6

as that felt like the safe, known version the plugin currently supports. Once we get through updates in #3 I expect this to bump to 7.4 or 8.x.