texvccheck is no longer distributed

[vroland]

To sanitize LaTeX-input we currently use texvccheck. But it is not only no longer used and developed by MediaWiki, but now also missing from the math plugin sources.
This means our build scripts cannot build the dependencies from scratch any more and we had to fall back on binary distributions for texvccheck.

We need to discuss possible replacements for texvccheck. Some desirable properties I could think of would be:

• useful error messages with position information
• easy to integrate with the linter and exporter
• possible to restrict to a strict subset we want to allow in MFNF
• normalize formatting and some semantic checks