Update optimization plugin

[tjheffner]

This should fix the extended lag seen at "92% chunk asset optimization UglifyJSPlugin" when compiling an app for prod.

This has been a common annoyance for webpack users lately, due to some unoptimized code for node 6 compatibility deep in uglifyJS
webpack-contrib/uglifyjs-webpack-plugin#272

one of the webpack maintainers recommends using the terser-webpack-plugin, which is another minify/uglify tool that will be the webpack standard in v5, but won't be updated/required for v4 due to some breaking changes.
facebook/create-react-app#4492 (comment)

some preliminary testing:

current status, master branch (uglifyJS)

build:pl
Version: webpack 4.23.1
Time: 326635ms

build:drupal
Version: webpack 4.23.1
Time: 303973ms

updated state, terser branch (terser-webpack-plugin)

build:pl
Version: webpack 4.23.1
Time: 34814ms

build:drupal
Version: webpack 4.23.1
Time: 29540ms

decrease in PL prod compile time: 291821ms or 4.86m faster
decrease in drupal prod compile time: 274433ms or 4.57m faster

[jryanconklin]

Makes sense to me.