Changed from uglifier to terser

[george-gca]

Changed from uglifier to terser for better ES6 support.

I created this PR as a draft since I don't know how to test these changes properly. I'd love some advice to better testing them so they would be considered to merge.

[digitalsparky]

Hi George,

Thanks so much I'll undoubtedly take a look at this and get it merged as soon as possible.
Please note that I do have a pain disability, so am unable to consistently work, so it may take a little bit.

Thanks so much for your support

Matt

[george-gca]

I didn't create tests for this or even tested in my machine because I have no idea how to set up a dev environment for gems. If you can help me or direct me to a good tutorial I am more than happy to guarantee that this actually works as expected. I just made the changes that seemed necessary for this to work properly.

Also if I manage to test this I could add the set up dev environment in another PR for your repo, no problem.

[ricky-ratnayake-trend]

Any progress on this? =) It looks like the current version of jekyll-minifier with uglifier is broken on Window WSL (platform ruby:x86_64-linux for me) and terser might help.