Enable LuaMetaTeX in ConTeXt tests and examples

[Witiko]

This PR enables LuaMetaTeX in ConTeXt tests and examples after #551.

[Witiko]

I wanted to test the new support for ConTeXt LMTX but I am still seeing some issues, namely the lack of support for the KPathSea library in LMTX, even in TeX Live, see the CI failure from this PR:

make FAIL_FAST=true test
make -C tests
make[1]: Entering directory '/__w/markdown/markdown/tests'
find testfiles/ -type f -name '*.test' -exec ./test.sh  {} +

Creating a Python virtual environment in /__w/markdown/markdown/tests/test-virtualenv.
2025-02-21 10:09:14,197 Running tests for 836 testfiles.
2025-02-21 10:09:14,197 Will fail at first error.
Testfile testfiles/regression/github/issue-508-fancy-lists.test:

  Some commands produced non-zero exit codes:
  - Command context [...] --luatex [...] test.tex [...] exited successfully.
  - Command context [...]          [...] test.tex [...] produced exit code 1.
  
  Some commands produced unexpected outputs:
  - Command context [...] --luatex [...] test.tex [...] produced expected output.
  - Command context [...]          [...] test.tex [...] produced unexpected output with the following diff:
  
    [...]

make[1]: *** [Makefile:13: all] Error 1
make[1]: Leaving directory '/__w/markdown/markdown/tests'
make: *** [Makefile:213: test] Error 2

Looking into the file test.log, I see the following:

lua error       > lua error on line 5 in file ./test.tex:

token call, execute: ...live/2024/texmf-dist/tex/context/base/mkiv/l-sandbox.lua:180: module 'kpse' not found:
	no field package.preload['kpse']
	no file '/usr/local/share/lua/5.5/kpse.lua'
	no file '/usr/local/share/lua/5.5/kpse/init.lua'
	no file '/usr/local/lib/lua/5.5/kpse.lua'
	no file '/usr/local/lib/lua/5.5/kpse/init.lua'
	no file './kpse.lua'
	no file './kpse/init.lua'
stack traceback:
	[C]: in upvalue 'requiem'
	...live/2024/texmf-dist/tex/context/base/mkiv/l-sandbox.lua:180: in function <...live/2024/texmf-dist/tex/context/base/mkiv/l-sandbox.lua:165>
	(...tail calls...)
	[ctxlua]:2: in main chunk
 1     % Load the package.
 2     \startluacode
 3     local kpse = require("kpse")
 4     kpse.set_program_name("luatex")
 5 >>  \stopluacode
 6     \usemodule[t][markdown]
 7     
 8     % Load the support files.
 9     \setupmarkdown [
10       eagerCache = false,
11       import = {
12         witiko/markdown/test = snippet as testsnippet,
13       }
14     ]
15

This is perhaps to be expected: Even if KPathSea is available for LuaMetaTeX in TeX Live, we apparently need to use the predefined object optional.kpse instead of require("kpse") according to luametatex.pdf. Regardless, KPathSea is unlikely to be widely available in ConTeXt standalone and therefore, we shouldn't rely on it.

[Witiko]

@andreiborisov: Can you please test that you can compile the example file examples/context-lmtx.tex using LMTX and version 3.11.0 of the Markdown package for TeX? You will need to remove the following part that loads KPathSea:

\startluacode
local kpse = require("kpse")
kpse.set_program_name("luatex")
\stopluacode

Furthermore, you will also need to copy all dependencies to the directory examples/. As discussed in #402 (comment), this includes at least the following files and packages from CTAN:

1. The expl3-generic.tex plain TeX file from l3kernel
2. The lua-uni-algos Lua package
3. The lt3luabridge plain TeX package
4. The tinyyaml Lua package

Please, let me know how that goes!

If you manage to compile the example document, I will use your feedback to hopefully have the CI running the LMTX tests and compiling the LMTX examples in the next version, to be released by the end of March at the latest. 🤞