feat(treeview): add experimental controllable API

[janhassel]

Ref #14451 / Point 1

This PR introduces an experimental controllable API for the TreeView to address point 1 mentioned in above linked discussion.

A user must opt-in via the feature flag enable-treeview-controllable.

Changelog

New

• feature flag enable-treeview-controllable
• TreeView props.onActivate to pass-through node's onToggle events
• TreeNode props.defaultIsExpanded for uncontrolled mode

Changed

• With above mentioned feature flag, the states selected and active can be controlled or uncontrolled based on the user's choice

Testing / Reviewing

• Storybook
• Test suites

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@tay1orjones I wanted to implement the feature flag approach you shared a while back for this change. Is there any naming guidance for these yet? I avoided v12 directly and essentially followed this structure:

enable-[component]-[feature]

Let me know what you think!

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[janhassel]

@tay1orjones @emyarod Let me know if you have any questions around this!

[alisonjoseph]

This LGTM! @emyarod do you have a chance to give this a review before we merge?