Changing EUI to use the lodash fork and forcing yarn usage

[kobelb]

Summary

Updating lodash to our our private fork. This requires us to use yarn instead of npm because of the alias: syntax that we're using in the package.json to do so transparently. Otherwise, we'd have to update all references that import/require lodash to @elastic/lodash.

Checklist

~~ [ ] This was checked in mobile~~
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added
- [ ] A changelog entry exists and is marked appropriately
- [ ] This was checked for breaking changes and labeled appropriately
- [ ] Jest tests were updated or added to match the most common scenarios
- [ ] This was checked against keyboard-only and screenreader scenarios

[chandlerprall]

LGTM; pulled and ran yarn successfully

[chandlerprall]

for the package-lock.json conflict, I think we can simply delete that file

[teotoplak]

Preinstall check script for yarn usage no longer needed since the upgrade of lodash to v4 (yarn alias is no longer used)?

[chandlerprall]

Appears that way; I was able to npm install --ignore-scripts @elastic/eui without any issue.