Add remark-abbr to tests and re-record snapshots

[richardTowers]

The only difference in the snapshots seems to be the presence of \\ backslashes before quote characters.

I'm a little confused by the output, not being all that familiar with jest, but I suspect this is some minor difference in the way that rehype-stringify works and that we don't need to worry about it?

This is just laying a small bit of groundwork, before I attempt to get remark-abbr working with the new version of remark

See also #514

[StaloneLab]

I'm way too surprised that the tests work (apart from these backslashes, I'll check). It seems that actually the plugin might work out of the box on new remark versions: it relies only on unist-util-visit which operates only on AST and does not require anything special. I think you would have noticed if it was the case, what am I missing? Or maybe you wanted to use the low-level micromark API?

[StaloneLab]

Hum I see unrelated linting problems, I fixed on next, you should be able to rebase (I forced push so maybe cherry-pick is easier). On top of that, there are some other changes to be done which we do while transitioning to next. I think the easiest way is to merge and I'll do the changes right after. They are stupid changes anyway, purely workflow, not really relevant to what you are trying to achieve.