Add support for GitLab CI

[sk82jack]

Description

This PR adds support for using GitLab CI pipelines as an alternative to AppVeyor and Azure Devops.

Related Issue

Fixes #5

Motivation and Context

Adds an option for folks who wish to use GitLab CI pipelines

How Has This Been Tested?

I've been testing by creating test repositories. Initially I ran into some issues with case sensitivity:

The first one (see screenshot 1) was that it couldn't find the tests folder because it's PascalCase in the Plaster manifest but the folder provided from PowerShellBuild is all lowercase. I fixed this by changing the case in the Plaster manifest.

Once the tests folder was fixed I ran into a second issue (see screenshot 2) that it couldn't find the public and private folders within the module because in the psm1 file they are all lowercase but in the Plaster manifest they are PascalCase. I fixed this by changing the case of each folder in the psm1 file.

Please let me know if you'd prefer alternative ways of dealing with these issues.

Here is an example of a successful test after making the fixes above: GitHub repository GitLab CI/CD jobs

Screenshots (if appropriate):

Screenshot 1

Screenshot 2

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[devblackops]

This is great @sk82jack. Thanks for add this and for fixing the bugs!