Add US English spell checker/linter for markdown

We should have a US English spell checker. 

https://github.com/mdn/content/issues/373 changed all know UK English spellings to US English, as per MDN policy. However this is likely to rot. As discussed in #10787 we should also have an automated spell checker system in place for English content. 