Documentation: update install commands and babel config

[oandregal]

This PR fixes some problems I ran into when following the current guide. It also promotes the use of --save-exact as a good hygiene practice.

• It asked me to install a webpack CLI (so I included it in the install commands):

• It failed to compiled due to incompatible babel packages:

[gziolo]

I'm not quite sure why --save-exact solves this issue given that both webpack and @wordpress/scripts follow semantic versioning. However if that helps, I'm fine with the changes proposed.

[oandregal]

@gziolo --save-exact is not required and can totally be dropped from this PR. It's a personal preference that I thought it was worth promoting as a good practice. I'm fine removing that from the PR if that's controversial.

[gziolo]

It might be something about the dependencies included by @wordpress/scripts. It's hard to judge based on Gutenberg because there are thousands of dependencies listed. I assume this is going to be no issue as soon as we have the default Webpack config included in the scripts exposed by the package we offer :)

[oandregal]

The root problem seems related to babel-loader@8 requiring @babel/plugin-transform-react-jsx while babel-loader@7 requires babel-plugin-transform-jsx, which is a different thing, apparently. So depending on which version of babel-loader gets installed, the other dependencies need to be adjusted.

[gziolo]

Do you know what is using Babel 6 specific packages causing babel-loader@7 to be pulled in?

[oandregal]

@gziolo not sure I follow. Here's what I know:

• Installing babel-loader will pull the latest version (8). That version requires @babel/plugin-transform-react-jsx.
• If we wanted to use babel-plugin-transform-react-jsx we should explicitly install babel-loader@7.

I chose to go with the first option because that's what Gutenberg uses.

[gziolo]

• If we wanted to use babel-plugin-transform-react-jsx we should explicitly install babel-loader@7.

Why would you want to install it in the first place?

I chose to go with the first option because that's what Gutenberg uses.

Yes, this is highly expected.

[gziolo]

Let's update for the time being and wait until we can address #13712 to make it perfect.