[EuiMarkdownEditor] A way to opt out line-breaks plugin

[sakurai-youhei]

EuiMarkdownEditor defaulted line-breaks plugin to follow the "github.com Flavored" ^[1] GitHub Flavored Markdown through #5272. In the PR and the source issue #5141, opt-out option was discussed, but it was not implemented at the end.

Now, because of elastic/kibana#180576, I'd like to propose the opt-out option. I will open a PR to make the line-breaks plugin excludable in addition to the tooltip plugin.

An alternative is to filter out unwanted plugins from the ones returned by gettDefaultEuiMarkdownPlugins(), but this requires the filter to be able to identify plugins.

1. ^ The GitHub Flavored Markdown (hereinafter referred to as the GFM) Spec defines soft line breaks, but the github.com Flavored GFM parses a regular line break as a hard line break. The line-breaks plugin realizes a line-breaking manner in the github.com Flavored GFM.