Remove obsolete/broken native markdown class

[Omikhleia]

This PR is a clean-up proposal:

• Remove from the core distribution the markdown class
  • It shouldn't have been a class (... one would rather want a package, so as to use markdown in any other classes such as book, letter, etc. naturally)
  • It was quite broken and limited, so... Closes markdown class is broken #1336 (and I wouldn't call that a breaking change even, although it's a matter of taste)
• Remove the then-unused Lunamark "vendored" code
  • It's no longer needed by SILE core stuff
  • It was quite obsolete anyway.

Rationale:

• As noted, the previous solution was an incomplete / not really working PoC
• These type things are better addressed in 3rd-party packages without being an impediment for the SILE core engine - And this is what my markdown.sile is attempting - N.B. I wouldn't mind giving it back to be hosted on the SILE organization, once it's mature enough.

By the way, as a consequence, some old issues are no longer applying:

• Closes Markdown reader doesn't handle hard breaks #416
• Closes Extricate Lunamark from source tree #669
• (IMHO) Closes Best markup for poetry regions (Markdown→Sile) #408

Of course, behind that quite-needed "good bye, legacy markdown class!" proposal, they are due credits that shouldn't be ignored. So thanks @simoncozens, @alerque and others that had the idea of using Lunamark, making that early attempt and foster the idea.

[Omikhleia]

By the way, unless I am mistaken, it seems SILE's dependency on cosmo is no longer required if the "vendored" Lunamark library is removed. I have not addressed that here -- But if we want to do it too, please ping me and discuss before, if possible.
I am having issues with 3rd-party packages that have a dependency on "LPeg". Depending on my "build path" for SILE, (via my own local build, a Docker image, or PPA etc.), "luarocks" doesn't seem to see SILE's own Lpeg setup and rebuilds one, possibly a different version. Used or not, I have no idea yet. I have been lacking time to investigate).

[alerque]

Thanks for the heads up, and will do.

I'd also seriously consider getting markdown.sile promoted to this ORG or even back in core in conjunction with dropping this module, but haven't been able to investigate that yet.

[Omikhleia]

Merely rebased / forced push to also remove the LICENSE-lunamark file added in the meantime (#1686)

[alerque]

@Omikhleia I think you are right about cosmo being no longer needed here. I'll drop it, but I don't understand your issue with LPEG. Maybe a dedicated issue for that if it is something I might be able to help look into?

[alerque]

Targeting develop per #1864.