chore: replace uglifyjs with terser for minifying prod builds

[dbjorge]

Description of changes

Replaces the minifier we're using on prod builds from uglify-js to terser.

Terser is a more updated replacement for UglifyJS. Webpack 4 switched to using it as the default minifier in Nov 2018 (webpack/webpack#8392). The reasons we'd want to switch:

• Unblocks future updates to typescript output target; uglify only supports up to ES5 javascript, Terser supports more recent javascript as input
• Closer to the "normal" webpack happy path
• As of recently, more widely used (with webpack)

It is essentially a drop in replacement; we expect no customer-visible changes as a result of this update.

Pull request checklist

• [n/a] Addresses an existing issue: Fixes #0000
• [n/a] Added relevant unit test for your changes. (yarn test)
• [n/a] Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• Ran precheckin (yarn precheckin)
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS