add a codeowners file to dart-lang/labs

[devoncarew]

• add a codeowners file to dart-lang/labs

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[devoncarew]

@mraleph @jonasfj @sigurdm - please let me know if you're comfortable getting automatically assigned as reviewers for the indicated packages (given our SLO for these packages, it's fine to respond to a PR indicating that you won't have time for a review).

[sigurdm]

Should we also add @szakarias ?