Add devcontainer Configuration

[n7studios]

Summary

Adds a devcontainer.json configuration to facilitate running a dev container locally or in GitHub Codespaces, using WPVIP's configuration, which provides a ready made nginx, PHP, MySQL and WordPress setup.

Codespace example:
https://expert-space-spoon-r4qrj6p9g9cx799-80.app.github.dev/
https://expert-space-spoon-r4qrj6p9g9cx799-80.app.github.dev/wp-admin

• User: vipgo
• Pass: password

Codeception (for end to end testing) isn't yet configured, as this requires chromedriver to run in a separate process.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[noelherrick]

Let's do this! I tested this out and it works great