Choose DCO or CLA

As an OpenJS Foundation project, we need to adopt either the DCO or a CLA (ref: [IP_POLICY_GUIDANCE.md](https://github.com/openjs-foundation/cross-project-council/blob/main/IP_POLICY_GUIDANCE.md#4-adopting-the-dco-or-a-cla)).

DCO is annoying and hard. CLA is easier but potentially more of a barrier for some folks.

There's some existing tooling for each:
- https://github.com/probot/dco
- https://easycla.lfx.linuxfoundation.org/#/
- the preferred CLA tool for the OpenJS Foundation, which they will directly support. If we want to use another tool, we can but they won't be able to help support it. I assume we don't care about that if we choose a different tool.