Introduce GitHub Actions for CI checks to replace Travis

[kasparsd]

It appears that Travis runs have been disabled for this repository after the move to the WordPress GitHub org:

Builds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the Plan page to replenish your credit balance or alter your Consume paid credits for OSS setting.

https://travis-ci.com/github/wordPress/two-factor

So we need to move https://github.com/WordPress/two-factor/blob/f10c28943ec4d71d78e510a6184e19d2d2f5c71a/.travis.yml linters and test runners to GitHub actions.

[jeffpaul]

@desrosj any chance you've got easily repeatable actions from your prior work on migrating Core to using GH Actions that could be repurposed for the 2FA plugin?

[kasparsd]

This is a bit more involved since we also need to replace the wp-dev-lib functionality with standalone tasks for things like linting (using wp-scripts and WPCS for PHP) and running tests (using wp-env locally and GitHub Actions during CI). I've started working on this already.