Allow Chrome to load speech worker from CDN in a page from a file:// URL

[dpvc]

This PR fixes a problem with importing the worker code from a page the was loaded from a file:// URL with Mathjax coming from the CDN. The problem only seems to affect Chrome, which won't allow the cross-origin load, even though the CDN marks the script as allowed.

The solution is to use importScripts() rather than import() to get the worker code. That means the worker can't be loaded as a module, as importScripts() isn't allowed there.

The package.json file is no longer needed, and the worker doesn't need to be a module anymore (that could have been removed when the worker-pool was removed, as that was only needed for the iframe hack to the liteDOM).

The v4-lab needs a new way to detect the need for the sre-lab.js file, as the importScripts() can't load an ES6 module, and the compiled speech-worker.js is such a module. So we need to use import() in the lab. The ww-v4-lab branch in the MathJax-dev repository has the needed changes: the HTML adaptor is subclassed and the createWorker() method is overridden with one that uses import(), and also sets self.isLab = true to mark the fact that the worker is running int he lab. The worker now uses that value to determine whether to load sre-lab.js or sre.js, so that is a bit cleaner than having to test for the failure of SRE loading.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.71%. Comparing base (4d1e195) to head (e6b134d).

Files with missing lines	Patch %	Lines
ts/adaptors/HTMLAdaptor.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1290     +/-   ##
==========================================
  Coverage    86.71%   86.71%             
==========================================
  Files          337      337             
  Lines        83979    83979             
  Branches      4750     3124   -1626     
==========================================
  Hits         72826    72826             
- Misses       11130    11153     +23     
+ Partials        23        0     -23     

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[zorkow]

lgtm.