chore: parallelize terser runs

[soldair]

this is missing needed negative tests to make sure error output is useful and exit code is correct.

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• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
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Does this PR introduce a breaking change?

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[Toxicable]

If you want I can give you my thoughts on running under worker_threads to do it at the same time? I don't think ill be able to go onto that for about 3 weeks due to Angular Connect then some time off afterwards.

[soldair]

@Toxicable this is meant to be throw away for yours. it should be good enough for a not going to blow up people MVP but i didn't try to make myself happy with it. =)

that said i'm happy to have feedback anywhere just @ me

[alexeagle]

We should land this now so we can publish the first version of terser_minified and get feedback without eating all the RAMz on someones machine.
We can leave the terser parallel issue open to track the worker_threads which I think would make it faster and consume less resources

[alexeagle]

yay it's green!