[UB] Clarify handling of whitespace in verbatim formatting

[mhatzl]

The spec does not describe how whitespace must be handled in verbatim formatting.
Outside verbatim context, non escaped whitespaces are combined to one space. Even new lines are combined.

For verbatim formatting, this is not the intended behaviour.
Here, all non line-breaking whitespace should be kept as is.

Line-breaking whitespace should be turned into one space.
Otherwise multiline verbatim formatting would always result in multiline output, because it is not possible to unescape a new line.
If a new line is wanted in verbatim formatting, there is still the option to escape it using a backslash.

[mhatzl]

For information:

GitHub follows the commonmark spec (https://spec.commonmark.org/0.30/#inlines), where <code> tags are used for inline verbatim, but <pre><code> for verbatim blocks. To still preserve non line-breaking whitespace in inline verbatim, they recommend adding CSS code{white-space: pre-wrap;}.