Swedish retailer H&M is breaking into Milan Design Week with a new collection in collaboration with the award-winning interior designer Kelly Wearstler. Wearstler’s high-profile work, including the interiors of the Proper Hotels , and for celebrity clients like Cameron Diaz and Gwen Stefani, has earned her A-list status in the industry. Now, with a first-of-its-kind collection, H&M is bringing Wearstler’s high-end designs to Main Street. “The constraints were very real. Everything had to work within specific production and shipping parameters. But that actually became a creative driver,” Wearstler tells Fast Company over email.…