Worker threads fixes and SourceMap revamp

[edemaine]

Worker threads (CIVET_THREADS feature) was broken in a few ways:

• Errors weren't serialized, so we got the empty {} exception if there was an error
• errors option didn't properly modify the array
• SourceMap outputs were crashing when serializing into a worker response message (because of function properties), which caused an error (see below). This caused yarn test to fail (before Parallel testing via CIVET_THREADS (Mocha's --parallel) #1665), and also caused civet filename.civet to fail. 🙀

Internal API change: SourceMap is now a class. This seems cleaner overall, and helps with serialization for worker messages. But it changes the API:

• Instead of SourceMap(source) we need to use new SourceMap(source).
• lines etc. are properties instead of part of the data field. In code that deals with multiple versions of Civet (e.g. LSP), I included a fallback for the old format.
• source is now a property, not a function. I don't think we use it anywhere though.
• updateSourceMap needs to be binded

Also the ESM register plugin tells you which file is erroring (which has been an issue in the past).

Also a small fix so that CIVET_THREADS=0 yarn test disables Mocha's parallel mode (to bypass export CIVET_THREADS=4). Previously 0 was treated as truthy.

BREAKING CHANGE: Must update VSCode plugin to 0.3.27 and eslint plugin to 0.0.6

[STRd6]

Thanks! We're finally migrating away from the CoffeeScript circa 2011 coding style :P