This patch transforms Markdown soft line breaks into hard line breaks
when rendering to HTML.
The [CommonMark specification about soft line
breaks](https://spec.commonmark.org/0.30/#soft-line-breaks) specifies:
> A renderer may also provide an option to render soft line breaks as
> hard line breaks.
Refering to https://github.com/vector-im/element-x-ios/issues/1418, some
people are expecting to get soft line breaks rendered at hard ones.
This patch updates the Markdown test to include this conversion of soft
to hard line breaks. It includes a list and a code block, to ensure
not _all_ soft breaks are transformed into hard breaks; only the ones
we expect.
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