update imgui_md: wrap + block-spacing fixes

- avoid mid-word break when an inline span (bold/italic/...) starts
  near the end of a line ("D" / "ear ImGui Bundle")
- fix asymmetric P<->list vertical gap (P->list was visibly larger
  than list->P due to a redundant gap on the first list item)
- fix oversized gap after <summary> in <details> blocks (a stray "\n"
  raw-HTML chunk was being rendered as text)

Author: pthom