fix: Make softlinebreak public

[mrueg]

Trying this here, as I haven't received a response on this https://github.com/yuin/goldmark/pull/425

This would match https://github.com/yuin/goldmark/blob/90c46e0829c11ca8d1010856b2a6f6f88bfc68a3/ast/inline.go#L115 and https://github.com/yuin/goldmark/blob/90c46e0829c11ca8d1010856b2a6f6f88bfc68a3/ast/inline.go#L85