[6.x] Replace underscore with lodash

[jasonvarga]

This PR:

• Replaces underscore.js with lodash (lodash-es) since lodash is the more widely accepted standard because of performance, modularity, more battle tested, popularity, etc.
• Removes the global _ helper and uses ES imports to improve bundle size*.
• Removes a bunch of underscore/lodash usage with native js equivalents.
• Snuck in removal of importer.js which was remnants from v2. It's not used and I only noticed it because it used an underscore method.

* Ironically, using lodash is a little heavier, but at least it's more modern. We may replace more bits with native JS in the future that will decrease bundle size further.

The removal of the _ helper is a breaking change and means addons will have to import lodash themselves.

+import { filter } from 'lodash-es';

-const filteredStuff = _.filter(stuff, fn);
+const filteredStuff = filter(stuff, fn);

However you may be able to just use native JS for most things anyway now. (And probably should.)

-const filteredStuff = _.filter(stuff, fn);
+const filteredStuff = stuff.filter(fn);