Add remark-unwrap-images to @gridsome/transformer-remark default plugins

Summary

remark-unwrap-images remark plugin removes paragraph tags that wrap images.

Basic example

GitHub for remark-unwrap-image

Motivation

It seems like a good default plugin to add since paragraphs shouldn’t be wrapping images.

[sebtiz13]

@alexbrown for your solution more reliable 🙏

But personally I prefer if it’s can be an options of @gridsome/transformer-remark so I make PR #1290 for that with more possibilities to cover #1195

[alexwidua]

In case someone runs into the same issue, I did a quick Gridsome implementation of gatsby-remark-unwrap-images, you can find the repo here: gridsome-remark-unwrap-images.

It basically the same approach @sebtiz13 did. There is one caveat that images have to be placed in a seperate line with one line break to the next paragraph, otherwise the image still gets wrapped in a <p> tag (see example).