Harper Integration

[Harper](https://github.com/Automattic/harper) is a deterministic, offline english spelling and grammar checker. It picks up a lot of common mistakes! We should use this in CI + have a configuration so the language server can be used in local development, but not necessarily block PR merges on it. [`harper-cli`](https://github.com/Automattic/harper/tree/master/harper-cli) is a CI-friendly frontend for harper. It's not very mature, but it's worth looking into if it does enough to be useful.