Add localization for mhchem error messages.

[dpvc]

This PR adds localization for the mhchem package, for both English and German. It uses the error id and message thrown by the mhchem parser and translates them into the format needed for the new localized TeXError object. Some of the messages have data inserted into the message strings that has to be extracted to be used as a parameter to TexError, and one of the messages is re-using the [tex]/base message Extra close brace or missing open brace.

I've updated the test file for mhchem, but have not been able to get full coverage. The bond error message actually can never be produced, but I've included it in case the mhchem parser ever changes to permit it. I wasn't able to generate one of the two-parameter bug messages (they seem to be for actual bug conditions, so may not be producible). So the coverage for mhchem is not as complete as it was.

Note that if you add

    json: (file) => import(
      file.replace('/bundle/input/tex/extensions/', '/ts/input/tex/'),
      {with: {type: 'json'}}
    ),

to the lib/v4-lab.js file in the MathJax configuration's loader block (just after require: ... for example), then this will cause the json files to be loaded directly from the ts/input/tex/<package>/__locales__ directory, so you won't have to keep doing the copy script. Also note that

./components/bin/makeAll --copy --terse components/mjs/input/tex/extensions/<package>

will perform the copying to bundle for a given component, while

./components/bin/makeAll --copy --terse components/mjs/input/tex/extensions

would copy all of them.

[zorkow]

I believe you are registering the wrong locale folder.

[dpvc]

I believe you are registering the wrong locale folder.

Thanks for catching that. Copy-and-paste error.