feat: Tree multiple level loading support and only count "items" for collection size

[LFDanLu]

Closes RSP Component Milestones (view)

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Test Tree multiloading stories in RAC storybook. Also test all collections that support loading indicators (Table, Listbox, GridList, Tree, ComboBox, Picker, etc) and make sure that the virtualized items have the proper aria-rowcount/posinset/etc. Loaders shouldn't be counted as items/rows. Sanity check that other collections work fine virtualized/non-virtualized and that empty states/initial load states still render as expected

🧢 Your Project:

RSP

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation

[rspbot]

## API Changes

react-aria-components

/react-aria-components:UNSTABLE_TreeLoadingIndicator

-UNSTABLE_TreeLoadingIndicator <T extends {}> {
-  children?: ReactNode | ((UNSTABLE_TreeLoadingIndicatorRenderProps & {
-    defaultChildren: ReactNode | undefined
-})) => ReactNode
-  className?: string | ((UNSTABLE_TreeLoadingIndicatorRenderProps & {
-    defaultClassName: string | undefined
-})) => string
-  style?: CSSProperties | ((UNSTABLE_TreeLoadingIndicatorRenderProps & {
-    defaultStyle: CSSProperties
-})) => CSSProperties | undefined
-}

/react-aria-components:UNSTABLE_TreeLoadingSentinel

+UNSTABLE_TreeLoadingSentinel <T extends {}> {
+  children?: ReactNode | ((TreeLoadingSentinelRenderProps & {
+    defaultChildren: ReactNode | undefined
+})) => ReactNode
+  className?: string | ((TreeLoadingSentinelRenderProps & {
+    defaultClassName: string | undefined
+})) => string
+  isLoading?: boolean
+  onLoadMore?: () => any
+  scrollOffset?: number = 1
+  style?: CSSProperties | ((TreeLoadingSentinelRenderProps & {
+    defaultStyle: CSSProperties
+})) => CSSProperties | undefined
+}

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