Remove markdown-spellcheck and switch to retext

[Elchi3]

I've noticed that marked is still in our dependency tree (used by markdown-spellcheck) and I've also spend some time to further read about the package universe unified that we've started to rely on. It is actually great and has tons of format and language processing packages! Really like that.

So, I'd like to propose to remove my initial spellcheck implementation (i.e. remove the markdown-spellcheck package) and start using retext and its plugins.

https://github.com/retextjs/retext is the parent package https://github.com/remarkjs/remark-retext can get you from remark to retext

Just like remark, retext comes with many plugins, but for language processing! So, not only it does spell checking, but also it has things to do further language checking like avoiding "guys" or checking if "a" and "an" are used correctly. Wow! https://github.com/retextjs/retext-spell https://github.com/retextjs/retext-equality https://github.com/retextjs/retext-indefinite-article List of language processing plugins: https://github.com/retextjs/retext/blob/master/doc/plugins.md

I'm impressed how much validation our writing could get :)

[peterbe]

I'm impressed how much validation our writing could get :)

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