TreeView: Retain expanded/collapsed state

[silvalaura]

Describe the bug
We should improve how TreeView retains the collapsed/expanded state of each item

To Reproduce
Steps to reproduce the behavior:

1. Navigate to https://storybook-preview-dev--upbeat-sinoussi-f675aa.netlify.app/?path=/story/treeview--complex&args=preselectedItems[0]:!undefined;preselectedItems[1]:!undefined;preselectedItems[2]:!undefined;preselectedItems[3]:!undefined;preselectedItems[4]:!undefined;preselectedItems[5]:!undefined;preselectedItems[6]:!undefined
2. Expand Chapter 3: Pudding
3. Collapse Part 1: Introduction, then expand it
4. Notice that Chapter 3 is collapsed
5. Now, collapse Chapter 5: Wafer
6. Collapse Part 2: Candy powder, then expand it again
7. Notice that Chapter 5: Wafer is expanded

In this example,
initialExpandedItems : [0 : "pt1"1 : "pt1ch1"2 : "pt2ch5.1"]

So it's trying to maintain the state based on the initialExpanded instead of latest state

Expected behavior
We expect it to remember the state it was in before I collapsed the parent, regardless of what it was set to when the page loaded.

So in the example above, Chapter 3 should still be open. Then, Chapter 5 should be collapsed. At least until the whole page is refreshed or something like that.

Additional context
This shows a good example of the expected behavior: https://vuetifyjs.com/en/components/treeview/#open-all