This creates a smaller state machine helper type for softness coalescing, which does not own the resulting nodes. While this creates a bit more duplication in stack and par builder, it makes it a lot easier to integrate additional logic into the paragraph builder.
Furthermore:
- Line breaks are now "hard", that is, not coalesced with each other.
- Text nodes with equal style are now merged allowing for example `f{}i` to form a ligature.
- Only add line spacing between lines. Previously, line spacing was added below
every line, making `#box[word]` higher than just `word`.
- Compute box height of text as `ascender - descender` so that the full word is
contained in the box.
