This change improves the terraform-provider code by supporting Windows in a more consistent/better manner.

This is done by:

• ensuring line-endings are disregarded when considering terraform state,
• ensuring path separators are standardised when uploading to Vercel (Closes "vercel_project_directory" data source does not parse the path properly when deploying on Windows. #50) and when considering terraform state, and,
• adding windows-latest as an OS to run tests against as part of CI.

Additional changes:

• I've added a concurrency section to the test CI workflow. This will prevent outdated tests running if multiple pushes are done in quick succession, and will cancel old CI runs.