Add existing contributors as CODEOWNERS

Currently only one person can merge MRs in this project and that's creating a bottleneck.

Adding a codeowners file starts the process of adding additional maintainers in a relatively informal way and with few expectations on the individuals listed.

If this change is merged, code-owners will be assigned to the compiler, npm module, and repl. This will be combined with MR approval rules on the project that require at least one approval from a relevant code-owner for each change.