Redesign the packages pages to include installation cmd directly on the first page

[ebenjs]

URL: https://pub.dev/

It will be nice to redesign the packages pages to include directly the installation command just like npm or crates.io do. Sometimes we just need to copy quickly this command and paste it into our terminal to install the package. We don't need to explicitly click on the install tab before being able to see the package installation command.

[sigurdm]

We have the button here that copies what you need to paste into pubspec.yaml:

Maybe we should also have a place for dart pub add <packagename>...

[ebenjs]

"Maybe we should also have a place for dart pub add "; yes this is what I'm talking about.

[ebenjs]

@sigurdm Can you have a look please ? #7305