Fix softbreak rendering within mixed namespace

[yhatt]

Reported in marp-team/marp#106.

Marp Core will convert line breaks always into <br /> tag for compatibillity with the classic Marp app. However, they may break rendering in the mixed namespace such as <svg> and <math> because <br> is available only in HTML namespace.

• markdown-it AST
• commonmark.js rendering

There is no problems in both parsers. We may have to consider the current namespace (explicit xmlns attribute and implicit <svg> and <math>) in softbreak token rendering.

[yhatt]

Transferred from Marp Core into Marpit. Patching on the level of framework should be better for the other Marpit flavored converters.