Verbatim and friends do not honor blank lines

[Omikhleia]

Same core issue as https://github.com/sile-typesetter/sile/issues/2128 but in addition

• (confirmed) We use silex.sile hacked typesetter, which is affected too = needing a a backport with a fix in silex and a dependency update
• (confirmed but see below) We use our own resilient.verbatim (style enabled) which is affected too
• (unconfirmed but likely) We have autodoc-resilient presumably affected too
• (unconfirmed) There are various hacks in markdown.sile that were used as workaround for this old issue and may potentially break after a fix. Maybe it will just be ok, but a non-regression is needed anyway...
  • explicit spaces in the user guide's examples would have to be checked...
  • and also the "naive" Lua syntax hightligher, if I remember correctly what it does (adding some zerohbox too if my memory is right)

[Omikhleia]

Oh interesting... resilient.verbatim is less affected than I thought... and has even some good code (see its "IMPLEMENTATION NOTE" in-code comment) that should be up'ed to SILE's regular verbatim, IMHO:

• honoring the fixed parts of the r/l skips... So code blocks in block quotes do what they should :)
• using hfillglue instead of some dubious 10000pt :)
• not cancelling baselineskip (which is a "small" part of the root issue...) :)
• leaveHmode in the scope of the temporary settings (less critical for SILE which doesn't have our styling overhead, uses a hard-coded 6pt initial glue and no final glue. Yep. Oh well.)

Go re·sil·ient :)

[Omikhleia]

(...) and also the "naive" Lua syntax hightligher, if I remember correctly what it does (adding some zerohbox too if my memory is right)

Indeed, this was always a bit dubious: https://github.com/Omikhleia/markdown.sile/blob/4c312906c400df54a32f85a520492fd8be063605/packages/markdown/commands.lua#L700

(Doh, using a ZWSP, not even a zerohbox, so my own memory of it was both right and wrong... :clown_face: )