fix: tree list container scroll control

[bsahitya]

Description

• Provide more control on the tree-list container in the shadow dom for the consumer applications.
• Using the part we can style the container as needed.

What's included?

• Add a part to the tree list container

Test Steps

• npm run storybook
• then open Tree list
• finally this

General Tests for Every PR

• npm run start still works.
• npm run lint passes.
• npm run stylelint passes.
• npm test passes and code coverage is not lower.
• npm run build still works.

Screenshots or link to StackBlitz/Plunker

[omaardev]

🎉 This PR is included in version 11.14.1 🎉

The release is available on:

• v11.14.1
• GitHub release

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