chore: update codeowners to project-based reviewers (#3253)

* chore: update codeowners to project-based reviewers

* chore: rm project stubs for projects without explicit filepaths

Author: stephl3

.github/CODEOWNERS

@@ -1,3 +1,7 @@
-# Global owners
+# project:chat
+chat/ @mongodb/lg-chat
 
-* @mongodb/leafygreen-ui-maintainers
+# project:time-input
+packages/date-picker @mongodb/lg-time-input
+packages/date-utils @mongodb/lg-time-input
+packages/time-input @mongodb/lg-time-input