Replace terser gem with its JS alternative

Problem

https://rubygems.org/gems/terser is a gem which wraps TerserJS to compress JS.

In !146348 (merged) we've removed the second to last gem which wraps JS and relies on execjs. terser would be the last one to remove to also remove execjs 🎉

Proposed solution

Replace terser gem with its JS counterpart.

Implementation guide

• Add TerserJS as NPM dependency
• Use TerserJS instead of terser gem
• Remove terser gem

Discussion

The following discussion from !146348 (merged) should be addressed:

• @splattael started a discussion: (+1 comment)

  Observation The gem https://rubygems.org/gems/terser still uses execjs which is also wrapping JS. I wonder if we could make use of TerserJS instead in the near future and get rid of execjs 🎉 🤞