Reimplement LaTeX setup snippets using lthooks

[Witiko]

Since version 2.10.0, the Markdown package has allowed users to define and invoke snippets that have a local effect on the behavior of markdown elements. By reimplementing snippets using @FrankMittelbach's lthooks instead of general TeX commands, it will be easier for different packages to modify one snippet without stepping on each other's toes.

[Witiko]

We should also ensure that \begin and \end{markdown} and {markdown*} invoke the env/markdown{,*}/{before,after} hooks.

[Witiko]

I am clearing the 2.11.0 milestone, since lthooks are only compatible with TeX Live 2021 and later. In order to support Overleaf, we will need to postpone using lthooks until TeX Live 2022 or 2023.

[Witiko]

After we start using lthooks, it may be benefitial to remove the compatibility code that works around the absence of lthooks.

[FrankMittelbach]

I am clearing the 2.11.0 milestone, since lthooks are only compatible with TeX Live 2021 and later. In order to support Overleaf, we will need to postpone using lthooks until TeX Live 2022 or 2023.

??? It was added to LaTeX 2020-10-01 (there are corrections in later release but the functionality should be there since then). Not sure what Overleaf is at at the moment.

[Witiko]

??? It was added to LaTeX 2020-10-01 (there are corrections in later release but the functionality should be there since then). Not sure what Overleaf is at at the moment.

@FrankMittelbach LaTeX hooks were unavailable in TeX Live 2020 on release and they are also unavailable in the TeX Live image that Overleaf is using at the moment (LaTeX 2020-02-02). Looking for a polyfill to \AddToHook{begindocument/end} left me empty-handed. The hook system looks neat, but it is not central to the Markdown package. If the onboarding cost is either losing TeX Live < 2021 or manually writing compatibility code, then that's a cost I'm not willing to pay at the moment and I'll postpone this issue.

[Witiko]

This is my mistake, really. When I created this issue, I assumed that there existed a package that served as a basis for the hook management system. As it turns out, a lot of the functionality is either a synthesis of several existing packages or brand new, which is exciting, but it also means that writing backwards-compatible code is difficult with the hooks.

[FrankMittelbach]

The concepts as such are brand new but when possible we reimplemented existing (more primitive hooks) with the new functionality. So yes, some of it can't be simply retrofitted except by rolling forward.

[Witiko]

I figured. We will probably start adding bits of \@ifl@t@r\fmtversion{2020-10-01}{ code with hooks }{ code without hooks } here and there, but will wait with the larger systems such as the setup snippets for a year or two.

[FrankMittelbach]

or perhaps even better:

\ifx\IfFormatAtLeastTF\@undefined  
  \def\IfFormatAtLeastTF{\@ifl@t@r\fmtversion} 
\fi

at the top and then use that (as it is the official interface for it).

[Witiko]

@FrankMittelbach Ok, thank you. Added in d3bdffb.

[FrankMittelbach]

more readable, less strange @s but of course unfortunately introduced only a couple of releases back, so will take a few years before one can use it without this guard at the top of the file.