Add prettier to ensure consistent formatting about bucklescript.github.io

[joealden]

Also, I would happily create a PR to add this to the project. I just need to know if it is something that would be wanted!

[joealden]

This would make it quite a bit easier for me to submit some PRs, as the really long lines make it difficult to check diffs in my editor.

[bobzhang]

hi @joealden , thanks for your interest in contribution. Currently there are not many contributors to this repo, as one of primary contributors, I am not motivated to learn a new tool(there are too many js tools already). But I don't mind accept PR which contains whitespace diffs, thanks