add a codeowners file (#330)

- add a codeowners file

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md)
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](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Most changes should add an entry to the changelog and may need to [rev
the pubspec package
version](https://github.com/dart-lang/sdk/blob/main/docs/External-Package-Maintenance.md#making-a-change).
- Changes to packages require [corresponding
tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing).

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

CODEOWNERS

@@ -0,0 +1,13 @@
+# Below are suggested reviewers for PRs to packages in this repository.
+#
+# These are suggestions; it is fine to have your changes reviewed by someone
+# else.
+
+/pkgs/blast_repo/              @kevmoo
+/pkgs/canary/                  @mosuem
+/pkgs/corpus/                  @devoncarew
+/pkgs/dart_flutter_team_lints/ @devoncarew @natebosch
+/pkgs/firehose/                @devoncarew @mosuem
+/pkgs/repo_manage/             @devoncarew @mosuem
+/pkgs/sdk_triage_bot/          @devoncarew
+/pkgs/trebuchet/               @mosuem