Multi-line effects

[Omikhleia]

A new approach derived from https://github.com/sile-typesetter/sile/pull/1334

• [x] Cleaner API
• [x] More robust re-boxing logic in standard mode (= paragraphing)
  • [x] Proper migrating content propagation (= footnotes)
  • [x] Correct handling of discretionaries
    • [x] Check for better approach
    • [x] Check that the so-daid "better" approach didn't butcher rskip/lskip margins
• [x] Does not break in horizontal-restricted mode (= inside other horizontal box)
• [x] Use in packages
• [x] Branch on SILE core for regression testing
  • [x] SILE manual fails. Investigate...
• [x] Extra robustness test: check it works as expected in a parbox too
• [x] Extra robustness test: check it works as expected in a boustrophedon

Caveat: It does further break the broken "pushback" logic (due to additional reboxing)

[Omikhleia]

As mentioned, links too can now span multiple line:

[Omikhleia]

Parboxing test (we rely on computed line ratios for the minimized case, so it's a good extra case)

[Omikhleia]

Also enjoying line-spanning underlines in a boustrophedon ('cause why not).

[Omikhleia]

Checking we didn't butcher the margins (lskip/rskip) in parbox and regular paragraph after more refactoring (https://github.com/sile-typesetter/sile/pull/1980)

EDIT: I really mean the (document) lskip and rskip "margins" here. Don't bother the unrelated "margins" frame in the screenshot (from resilient's layout)

[Omikhleia]

Extra tests with some of my stuff:

• [x] lefevre article: one link on a QR code
• [x] resilient manual (not the latest version but a copy used from previous sile-x regression testing)
  • [x] ToC links
  • [x] Lots of colors (with autodoc-resilient variant)
  • [x] markdown and djot manual (latest v1.5)
  • [x] underline and strikethrough
  • [x] one link on last page was long, overflowing the page frame - it should now be wrapped and still work.