Change Symbol to Token to avoid conflict with native Symbol in webpacked files. (mathjax/MathJax#3072)

[dpvc]

This PR changes the Symbol class name to Token so that it doesn't conflict with the native Symbol object in webpacked files. Webpacking of ESM files handles the exports from the component lib files as global variables, and that means the Symbol object will overwrite the native one, and since the webpack initialization code refers to Symbol before that const Symbol is set up, that cause a compile error. (The old CommonJS modules.export approach didn't use separate global variables, so didn't have that problem.)

Along with this, it renames SymbolMap to TokenMap, and changes variables called symbol to token, where appropriate.

Unfortunately, this touches a lot of files.

Resolves issue mathjax/MathJax#3072.

[zorkow]

I did not review all files in detail. I built, tested in the lab and with my own test page. It works fine.