Add CODEOWNERS file (#877)

Author: mosuem

.github/CODEOWNERS

@@ -0,0 +1,21 @@
+# 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.
+
+.github/ @dart-lang/dart-ecosystem-team
+.gemini/ @dart-lang/dart-ecosystem-team
+README.md @dart-lang/dart-ecosystem-team
+pkgs/args @munificent
+pkgs/async @dart-lang/dart-core-packages-team
+pkgs/characters @lrhn
+pkgs/collection @dart-lang/dart-core-packages-team
+pkgs/convert @dart-lang/dart-core-packages-team
+pkgs/crypto @dart-lang/dart-ecosystem-team
+pkgs/fixnum @dart-lang/dart-core-packages-team
+pkgs/lints @dart-lang/dart-ecosystem-team
+pkgs/logging @dart-lang/dart-ecosystem-team
+pkgs/os_detect @lrhn
+pkgs/path @dart-lang/dart-ecosystem-team
+pkgs/platform @lrhn
+pkgs/typed_data @dart-lang/dart-core-packages-team