Fix problem with sre-root

[dpvc]

This PR resoves a problem with how the worker.path option is computed. It needs to be a full path when used in node applications, and the ts/components/mjs/sre-root.ts file uses import.meta.url to determine it. But webpack converts that to the full unix path of where it was compiled, and so that path was being included in the component files like tex-chtml.js which is not desired. (It would get replaced when used in a web setting, but not in a node setting.)

The ts/components/mjs/root.ts file works similarly, but is replaced during webpacking by components/root-pack.js, and that needs to be done for sre-root.js as well. So I've added the needed sre-pack.js file and a webpack rule to do the replacement.

In addition, there was still a problem with producing a full path to the webworker file when the mathjax root is set to a node packlage like @mathjax/src/bundle, which is the most convenient thing to do in node applications that use MathJax components. This works OK when loading the components from source, but not when loading the bundled versions. So I've modified components/mjs/a11y/speech/speech.js to work harder to resolve the path for the worker. This now works in all the use-cases that I have tried (loading of bundled combined components, loading of combinded components from source, loading of individual bundled components, loading of individual compoments from source, direct loading of modules). So I think this takes care of all the worker path problems.

Now that the speech is in the worker, not MathJax proper, the components/mjs/a11y/sre/sre_config.js file, which had been setting up the path for the JSON files, is no longer needed, so I've removed that.

Finally, I changed the version number to 4.0.0-rc.2.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.69%. Comparing base (ef47836) to head (f7d8f6f).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1284   +/-   ##
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  Coverage    86.69%   86.69%           
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  Files          337      337           
  Lines        84107    84107           
  Branches      4753     4752    -1     
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  Hits         72919    72919           
  Misses       11165    11165           
  Partials        23       23           

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[zorkow]

lgtm.