Support unicode-display_width v2

[dduugg]

Adds support for v2 of unicode-display_width. This version emits a warning when requiring the no_string_ext version, since omitting String extensions is the default. To compensate, I've reverted to the normal require, but this will of course include String extensions when the gem version is < 2. (It may make sense to require the unicode-display_widthversion be >= 2, to avoid unintentionally using a conflicting String API, let me know which approach you'd prefer.)

The only use of this library appears to be an invocation of Unicode::DisplayWidth.of in lib/rubocop/cop/mixin/alignment.rb, so this should be compatible with the v2 library changes.

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[bbatsov]

Thanks!