Replace uglifyjs with terser

[j-maas]

terser does not require two passes. I tried out running the --compress and --mangle passes separately, but running one pass with --compress --mange yields the exact same byte count (just as it should be). (uglifyjs, even in the current version 3.8.0, still requires two passes.)

The compression is very close to uglifyjs. uglifyjs takes a 691.340 bytes elm.js to 147.094 bytes, while terser yields 149.607 bytes.

The reason I tried out terser was that uglifyjs does not handle modern JavaScript (newer than ES5), which lead to an issue in elm-pages. Terser does, is nicely documented, and easily replaces uglifyjs.

Supporting modern JS is not really relevant here right now, since the Elm compiler outputs ES5. But that terser only requires one pass makes explaining it easier.

[jhrcek]

The comment you're linking talks about uglify-es and is two years old.
uglify-js seems to be very much alive judging by the activity in the repository. I have nothing against your proposed PR, just bringing this up to prevent potential misunderstanding.

[j-maas]

Sorry, that is true! I got confused there. I will update the comment. Thanks for the correction!