Switch Rollup minifier plugin to rollup-plugin-terser

[daviwil]

In PR #4165, @bterlson recommended that I use rollup-plugin-terser instead of rollup-plugin-uglify to mitigate an issue where an ES6 class definition caused the minifier to throw an error about an unexpected token. @HarshaNalluru asked whether we should be using this everywhere, so this PR is an attempt to get us moved over to rollup-plugin-terser anywhere that rollup-plugin-uglify is being used. The keyvault libraries and core-http both use uglifyjs directly (they probably shouldn't) so I didn't update those at this time (but I can if folks feel strongly about it).

I've made changes for each library in individual commits so it's easy to drop any that we don't feel should be moved over!

[daviwil]

Noticed that core-auth didn't have the tree-shaking test fix that was introduced in #3997 so I added it here so that tests will run correctly.

[bterlson]

LGTM!

For history: I would have definitely adopted terser back when I was working on Storage a year ago, but it was riddled with bugs. Since we required the es5 target, terser really wasn't needed. Now that we target es6, it is, so I'm glad things seem to be working much better now!

[XiaoningLiu]

@bterlson Storage still targets es5, which bugs you mentioned by changing to terser?

As the promise to support IE in current V10 GA storage SDKs, changing to es6 seems not appliable.

[daviwil]

Since tests are passing, I think that the terser-derived output must be working fine. I'll go ahead and merge this now so that all of our packages are configured consistently. If we notice any issues with specific packages we can revert them individually to use rollup-plugin-uglify.

[bterlson]

@XiaoningLiu I don't recall anything affecting ES5 patterns so I think this upgrade is fine.