Run spelling check in github action

[lianzhao]

I think it's better to have some spelling check tools integrated into github action.

A spelling check tool should

• be able to check .md & .json files
• be able to check English at least, better to have other languages support
• be able to run locally in dev env
• be able to run in github action
• be configurable, e.g. to ignore words like 'Hoid' or 'Roshar'

AFAIK, cspell

• works as cli
• works as vscode extension
• works as github action
• could build specialized dictionaries. Maybe we can build a Cosmere dictionary from coppermind using Mediawiki API?