importmap pin/update removes preload: false

[mhenrixon]

When I run importmap pin @turbo-boost/commands it changes the importmaps.rb line from:

pin "@turbo-boost/commands", to: "@turbo-boost--commands.js", preload: false # @0.0.16

To:

pin "@turbo-boost/commands", to: "@turbo-boost--commands.js" # @0.0.16

I believe that it should keep any existing preload: false

[dhh]

There's no such guarantee of persistence when using the pin command. It's a fine line here, but we can't and won't turn this into a full-blown package manager, so going to call this out of scope at the moment.