Update version to beta.3

[dpvc]

This PR includes several changes needed for version 4.0.0-beta.3. These fixes include:

• Using the correct root path for webpacked node-main.js (for when you use require("mathjax") or import "mathjax" or the same for mathjax-full.
• Replacing mjs/root.js in the webpacked combined configuration files with a new components/root-pack.mjs that does the same thing as __dirname does in CommonJS webpacking (it replaces it with the directory /). This is not ideal, but it prevents the inclusion of the directory name where the webpacked file was created (e.g., /home/travis/mathjax/MathJax-src/...). The v3 versions had a broken default directory in node as well, so this is no worse than in the past.
• Fixing some comments (in particular some copyright dates that should not have been ranges).

[zorkow]

Please run npm install before merging, so package-lock.json will also be updated to beta.3.

[dpvc]

Yes, I'm in the process of building the beta.3 font packages, and was going to update the package-lock.json after they are in place, but you beat me to it. Almost there.