fix(markdown-widget): apply list item style on each block in a selection

[bytrangle]

fixes #5654

Summary
When selecting multiple paragraph blocks and click List button, the current behaviour is to turn them into one list block which contains one list item, no matter how many paragraph blocks are selected.

This PR changes that behavior to make Netlify CMS editor more in line with the convention in most rich text editors. Each paragraph block in the selection will be wrapped with a respective list item. Then all the list items will be wrapped with one unifying list block.

HTML before clicking List button:

<p>Item 1</p>
<p>Item 2</p>
<p>Item 3</p>

HTML after clicking List button:

<ul>
  <li>
      <p>Item 1</p>
  </li>
  <li>
      <p>Item 2</p>
  </li>
  <li>
      <p>Item 3</p>
  </li>
</ul>

Test plan
TODO:

• Write a test to check if all bottom-most blocks in a selection are turned into list items when clicking List button
• Write a test to check if all list items in a selection are turned back into paragraph blocks when clicking List button

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• Code is formatted via running yarn format.
• Tests are passing via running yarn test.
• The status checks are successful (continuous integration). Those can be seen below.

[bytrangle]

Hi @erezrokah,
I recently changed my OS and had to set up Yarn from scratch. After running yarn bootstrap, I ended up with more than 2000 pending changes including changes to the following Yarn-related files:

• addition of a folder name .yarn. There's a topic on Yarn saying that it should be committed, but really?
• yarn.lock
• yarnrc.yml
• netlify-cms-proxy-server/package.json with the addition of this line: `"bin": "./dist/index.js".

What files should I commit?

[erezrokah]

Hi @erezrokah,
I recently changed my OS and had to set up Yarn from scratch. After running yarn bootstrap, I ended up with more than 2000 pending changes including changes to the following Yarn-related files:

• addition of a folder name .yarn. There's a topic on Yarn saying that it should be committed, but really?
• yarn.lock
• yarnrc.yml
• netlify-cms-proxy-server/package.json with the addition of this line: `"bin": "./dist/index.js".

What files should I commit?

Which version of yarn are you using? Maybe this is related to yarn v1 -> v2 changes?

[bytrangle]

It is 1.23.0, after I had run yarn set version classic because lerna bootstrap doesn't work on Yarn v2.

[erezrokah]

It is 1.23.0, after I had run yarn set version classic because lerna bootstrap doesn't work on Yarn v2.

So I'm not sure. I'm using 1.22.5 and our CI uses 1.22.11, so maybe try aligning your version to one of those. If you'd like you can commit the files to a separate branch and I'll try to see what happens when I run yarn bootstrap on that branch.

On another note, I've added two commits, one a refactoring and one a fix to set the correct list type.

[erezrokah]

This looks 🚀 Those new tests are 💯

[bytrangle]

Thank you for all those correction. I learned a lot ✨.