Bump up the major version

Enabling Unicode slugs is a major change
in the behavior of the library. Therefore,
updating the major version of the library
from 2.0.0 to 3.0.0.

Author: sentinel-1

pubspec.yaml

@@ -1,5 +1,5 @@
 name: slugify
-version: 2.0.0
+version: 3.0.0
 description: >-
   Converts a string into a slug.
   Useful for URLs, filenames, IDs, and more.