[chore] update scripts to detect macos or linux for GNU sed usage

[adrielp]

Changes

Adding some basic OS detection in scripts and make due to the difference between sed on MacOs and Linux. Namely, gsed (GNU Sed) will be used on MacOs. The contributing guide documents the install under pre-reqs and gives the brew bundle command as an optional way to install gsed on Mac. This is simply a quality of life improvement for those on Mac.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [x] schema-next.yaml updated with changes to existing conventions.