Support grammarly like grammer checking

[Yukaii]

related #647 #389

We might implement this by using languagetool API on a self-hosted LanugageTool instance.

• https://github.com/languagetool-org/languagetool
• https://github.com/markfullmer/grammark
• https://github.com/btford/write-good
• https://github.com/mikpanko/expresso

[Yukaii]

There's an unofficial Grammarly VSCode extension now, can see how it uses Grammarly authentication API and retrieve diagnosis info with WebSocket. So it's totally possible to do Grammarly integration for CodiMD. (But also have to bypass CORS request through backend or some proxy mechanism)

• https://github.com/znck/grammarly/blob/master/src/shared/grammarly-auth.ts (Authentication)
• https://github.com/znck/grammarly/blob/master/src/shared/grammarly.ts (WebSocket requests)