Use cross-spawn instead of child_process and introduce _W32 placeholders

[jlelong]

Related to the tests and discussions in #2010.
It must be extensively tested before merging (paths with /, \\, spaces and also custom recipes using system commands).

Close #2010

Edit: Summary of the PR

• All calls to child_process.spawn are replaced by cross-spawn.spawn, which takes care of escaping issues in a cross platform way.
• We introduce new placeholders %DOC_W32%, %DOC_EXT_W32%, %DIR_W32%, %OUTDIR_W32%, which are normalized so that they use \\ as the path separator on Windows. Placeholders without the _W32 suffix always use / as the path separator. On Unix platforms, placeholders with and without the _W32 suffix have the same value.

[jlelong]

@tamuratak while I was adding more tests, I realised that the issue on /vs \ on Windows is deeper than I thought. See https://stackoverflow.com/a/10526678 for a series of (non-) working examples.

I finally figured out that the tool

      {
          "name": "copyPDF",
          "command": "copy",
          "args": ["%OUTDIR%/t.pdf", "%OUTDIR%/b.pdf"],
          "env": {}
      }

works fine as long as %OUTDIR% = %DIR% . Remember that the cwd of the spawned command is %DIR%. The issue arises when the dirname of the file to be copied uses / and does not match the cwd.

I think that instead of introducing _W32 placeholders, we should add a normalizing flag for each tool. Clearly, LaTeX commands must use / as separators regardless of the os, but for native commands normalising the args is simpler and even required in some cases. Now, the question is how to achieve this with breaking compatibility.

EDIT: Forget about that, this is far too intrusive and it may cause a lot of issue. I will instead test an other strategy:

• Introduce _W32 placeholders that are always normalized
• Expand all other placeholders to always use /
• Do not further alter the arguments of spawn commands. The user is responsible for using the correct path separator with the correct placeholders. For instance, we would not correct %OUTDIR_W32%/out.

[jlelong]

See https://github.com/James-Yu/LaTeX-Workshop/wiki/Compile#placeholders for the definitions of the new placeholders.