[13.x] Add method to detect if app is running in CI environment

[ziadoz]

This adds a runningInContinuousIntegration() method. It's useful for skipping flaky tests that require investigation, or configuring CI specific setup before tests.

It checks the CI environment variable is true, as it seems standard across various services:

• GitHub: https://docs.github.com/en/actions/reference/workflows-and-actions/variables#default-environment-variables
• GitLab: https://docs.gitlab.com/ci/variables/predefined_variables/#variable-availability
• BitBucket: https://support.atlassian.com/bitbucket-cloud/docs/variables-and-secrets/
• Travis: https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
• Circle: https://circleci.com/docs/reference/variables/

[github-actions]

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[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!