Add JSON Schema intellisense for `harper-ls` settings when editing `Zed/settings.json`

[Greenheart]

Motivation

This would make it easier to configure the harper-ls by making the options easier to discover. This would also help catch errors like misspelled options or invalid/old config values.

Current workaround:

For now, it works well to use the official harper-ls docs to customise configuration: https://writewithharper.com/docs/integrations/language-server#Configuration