edited .separatedBy() to return a list by default.

[decheverri123]

Description

.separatedBy() method kept throwing error getting Iterable<Widget> can't be assigned to parameter type List<Widget>.
This change updated the .separatedBy() method to return a list by default so examples like this which were documented on the pub page will work.

body: Center(
       child: Column(
           children: [
            Text("Hello"),
            Text("Bye"),
            Text("Ca va")
        ].separatedBy(SizedBox(height: 20)),
),

Some tests were also added to insure functionality.

Issue link

If your PR closes an issue, use the closes keyword followed
by its link, for example:

closes https://github.com/nank1ro/awesome_flutter_extensions/issues/{ISSUE_NUMBER}

replacing {ISSUE_NUMBER} with the number, like: 1

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• [ x] 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [x ] 🧹 Code refactor
• [x ] 📝 Documentation
• 🗑️ Chore

[nank1ro]

Thank you for your PR. I really appreciate it. Just commented two things but it's good

[nank1ro]

Just bump the lib version to 1.1.1 and update the CHANGELOG please and I merge it

[nank1ro]

LGTM! 🚀 Thank you!