Official support for Harper grammar checker

[saddle116]

Harper (https://writewithharper.com/) is a grammar checker, similar to languagetool or grammarly based on the lsp-protocol.

Currently, doom supports languagetool, which requires a java runtime and is less extendable. Harper is a foss alternative that is magnitudes faster and allows for editable plain-text dictionaries and rule sets.

It already has support for emacs via lsp/eglot, so a module or option under :checkers should be trivial

https://writewithharper.com/docs/integrations/emacs