Recognize *.ghe.com hosts as GitHub Enterprise Cloud

[mcomans]

Summary

• Add host.endsWith(".ghe.com") to the github branch of hostToProvider(). Suffix match (not includes) so attacker-controlled hostnames like evil-ghe.com.attacker.com don't
  false-positive.
• Add unit tests to cover positive (acme.ghe.com, tenant-name.ghe.com) and negative (bare ghe.com, substring) cases.

Context

We hit this when running the action against a repo hosted on GitHub Enterprise Cloud with data residency (hostname under *.ghe.com). hostToProvider() returned null for the host, so the Linear API rejected the sync with:

Variable $input got invalid value null at input.repository.provider;
Expected non-nullable type String! not to be null.

As a quick workaround we rewrote the git remote to swap the *.ghe.com host for github.com so the URL would hit the existing github branch in hostToProvider(), however that results in the links to the repository from Linear not working correctly. GHE Cloud should be functionally equivalent to github.com for what the CLI cares about, e.g. the same PR-merge commit conventions hold (Merge pull request #N from owner/branch), so extractBranchNameFromMergeMessage and the rest of the pipeline work unchanged.

About GitHub Enterprise Cloud with data residency and GHE.com domains