fix(treeview): update Treeview margin and focus border to use Spectrum tokens

[jonnykry]

Description

No specific issue for this change. While working on a Treeview implementation, I noticed two lines of the CSS where we could use Spectrum tokens.

How and where has this been tested?

Tested locally by:

• Adding the changes to my existing Tree View implementation
• Running spectrum-css Storybook locally

Validation steps

Validate visual Tree View before and after, specifically for margin and focus border visual differences. They should be identical.

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

• The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

• VRTs have been run and looked at.
• Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

Screenshots

To-do list

• I have read the contribution guidelines.
• I have updated relevant storybook stories and templates.
• I have tested these changes in Windows High Contrast mode.
• If my change impacts other components, I have tested to make sure they don't break.
• If my change impacts documentation, I have updated the documentation accordingly.
• ✨ This pull request is ready to merge. ✨

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[castastrophe]

Hello! I really appreciate this contribution, however, due to some of the rules in our system about when to use certain tokens, I'm not sure if we'll be able to accept this change at this time. Please don't hesitate to reach out with questions!

[castastrophe]

Though this resolves to 1px, we don't mix component-level tokens like this in our system. We'd need to have a --spectrum-tree-view-divider-thickness value (or something of the like) added to the token data in order to update this.

[castastrophe]

Thanks for tackling this! Unfortunately, the spacing-300 value does not resolve to the same rendered result as 1em since the ems are relative values to the typography. This was an intentional choice made for tree view to ensure spacing mirrors the sizing of the content.