chore: add more detailed tree view samples

[bheston]

Added more detailed tree view samples including end icons and flat

Description

Added end slot icons to the Storybook fixtures.
Added a "Flat" sample to the Explorer config.

Motivation & context

I needed to see how these components rendered these behaviors by default and found there weren't any current examples.

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.