Use TEXMF_OUTPUT_DIRECTORY to set \markdownOptionOutputDir

[Witiko]

As discussed on our matrix.org, in KPathsea from TeX Live 2024, the option --output-directory from engine invocation is available in TEXMF_OUTPUT_DIRECTORY environmental variable. We can use this to set \markdownOptionOutputDir based on --output-directory. After we have added support for TeX Live 2024 (pretest) in #404, we should:

• Automatically set \markdownOptionOutputDir based on TEXMF_OUTPUT_DIRECTORY if it's set or fall back to current working directory otherwise.
• Deprecate \markdownOptionOutputDir and make a comment that says that we have to drop support for TeX Live 2024 after we remove it in the next major release.

Here is a more detailed plan of action:

• Move the loading of the lt3luabridge library before defining the outputDir plain TeX option.
• Use \sys_if_shell_unrestricted:TF to decide whether we should try to determine TEXMF_OUTPUT_DIRECTORY.
• Use \luabridge_now:n { print(os.getenv("TEXMF_OUTPUT_DIRECTORY") or ".") } to determine TEXMF_OUTPUT_DIRECTORY.
• Store the result of \luabridge_now:n in \l_tmpa_tl and use a new variant \@@_add_plain_tex_option:nnV to set the default value from \l_tmpa_tl.
• Mark \markdownOptionOutputDir as deprecated in technical documentation.