Add GitHub community standards files (and others expected by plugins on GitHub)

[jeffpaul]

• Code of conduct, eg. https://github.com/10up/.github/blob/trunk/CODE_OF_CONDUCT.md
• Contributing, eg. https://github.com/10up/.github/blob/trunk/CONTRIBUTING.md
• License, eg. https://github.com/10up/insert-special-characters/blob/develop/LICENSE.md
• Security policy, eg. https://github.com/10up/.github/blob/trunk/SECURITY.md
• Issue templates, eg. https://github.com/10up/.github/tree/trunk/.github/ISSUE_TEMPLATE
• Pull request template, eg. https://github.com/10up/.github/blob/trunk/.github/PULL_REQUEST_TEMPLATE.md
• dependabot.yml focused on security-related updates, eg. https://github.com/woocommerce/woocommerce-square/blob/trunk/.github/dependabot.yml
• composer.json
• package.json