[EuiMarkdownEditor] Add remark-breaks and line break plugin

[i-a-n]

Summary

This adds the remark-breaks plugin to the default list of plugins for the MarkdownEditor. Now, unless disabled, all single-line linebreaks in the MarkdownEditor will translate to literal line breaks in the preview mode.

Note that multiple consecutive line breaks will still result in only two line breaks, as it does today, so there are no concerns about runaway line breaks making things look weird. Behavior mimics GitHub's comment box.

Checklist

• Added or updated jest and cypress tests
• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

Resolves #5141

[Opening in Draft Mode since this is technically a change in behavior, and I'm not sure if this should be considerd a breaking change since the component is technically in Beta, so I'll be waiting for some feedback on that before merging.]

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5272/

[elizabetdev]

Thanks @i-a-n for opening this PR,

I also don't see this as a breaking change and we're saying since day one that the markdown editor "uses GitHub flavored markdown". So I guess this behavior should be expected.

For allowing to opt-out the plugin you can see how @chandlerprall did it here: elizabetdev/eui@90a669a#diff-d5fec82a5af6bfbe83ee2cee3b7aad4e015f98f57996b8f8eb703a14688efbc0R74.

And we should say in our docs that we can unregister this plugin in the following section:
https://eui.elastic.co/pr_5272/#/editors-syntax/markdown-editor#unregistering-plugins

And update the following demo to show how we can also unregister the line break plugin:

All the rest looks good to me! 👍🏽

[i-a-n]

thanks for taking a look @miukimiu .

I was going deeper into this one and found a few things:

1. this line breaks plugin I added is nearly identical in structure to the emoji plugin that's already there... it is imported in the same place (as a getDefaultEuiMarkdownParsingPlugins plugin).
2. the emoji plugin does not appear to be "unregisterable", like how tooltip can be unregistered thanks to @chandlerprall's work in the PR you linked.

I think this leaves us with a few directions we can go in, and perhaps I'll leave it up to you & the team which is the best option:

1. we can leave this line breaks plugin like it is now, keeping it the same as emoji: a default github-flavored markdown plugin that cannot be easily excluded
2. we can make line breaks a special exception, and give it another exclude: ['line-breaks'] prop like how we invented exclude: ['tooltip']—although it might be a bit odd that we can't easily exclude other parsing plugins like emoji in this fashion
3. we can make all default plugins "easily excluded", which will require minor refactors of each default plugin function, and document publicly all of the excludable plugins (I think there about 6? in total)
4. some other option I haven't considered

let me know what you think! happy to help with whatever the team decides.

[chandlerprall]

These changes LGTM;

Since we target github flavor markdown, adding this feels more like a bug fix than what we'd consider a breaking change, and with that inclusion I'd consider this a non-disableable plugin, as opposed to our addition of tooltip to the markdown syntax (option 1 in #5272 (comment)).

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5272/