Support parsing marks outside of a known parent node
Support parsing marks outside of a known parent node

FIX: [`DOMParser.parseSlice`](##model.DOMParser.parseSlice) can now
correctly parses marks at the top level again.

See https://discuss.prosemirror.net/t/copy-pasting-html-that-doesnt-contain-a-block-node-strips-marks-away/1305

ProseMirror/prosemirror-model