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.

Start by reviewing the repository's existing CI configuration and Harper's harper-cli entry point to determine whether it is mature enough for this use. Also investigate the requested language-server configuration for local development. Done means Harper checks run in CI without blocking merges and the local-development configuration is available.

Written by the indexing model from the issue text.