Improve handling of Markdown edge cases

[fleven-kds]

Markdown allows for some formatting issues within components. For example, it is possible to insert <p> tags within headlines by using a line break in Markdown.

[julrich]

We should be able to fine-tune this by using the following configuration option "Customize toolbar" for Markdown field types in Storyblok:
https://www.storyblok.com/docs/schema-configuration#markdown

But I guess the actually available options for Markdown fields would vary across different uses (e.g. <p> not allowed in Headline text, but being totally fine and needed in longer text in the Text or Image Text components). Maybe need to introduce another x-cms- option for this.