Add separate setting about line breaks for Marp preview

[yhatt]

Unlike CommonMark, it is Marp Core's default behavior to convert line break into <br /> tag. (https://github.com/marp-team/marp-core#marp-markdown)

VS Code can configure the behavior of line breaks by markdown.preview.breaks (false by default) and Marp VSCode is also following. But it may confuse by the difference of default line break setting when using the same Markdown in the other Marp tools.

It would be good if there was separated markdown.marp.breaks option (true by default).