Update to terser v5

[boris-petrov]

v5.0.0 of terser has been released.

This is important as 1) it's good to use the latest version of the production minifier and 2) nullish coalescing / optional chaining support will only be released in terser 5 which is needed to fix this other issue in ember-cli.

[nickschot]

Terser 5.2.x with support for optional chaining has been released now.

[nickschot]

terser.minify() is now async, so some work needs to be done to support it.

https://github.com/ember-cli/broccoli-uglify-sourcemap/blob/master/lib/process-file.js#L51
https://github.com/terser/terser/blob/master/CHANGELOG.md#v500-beta0

[rwjblue]

This has been released in https://github.com/ember-cli/broccoli-uglify-sourcemap/releases/tag/v4.0.0