feat: modifies `useHeadlessFlatTree` return signature to include `getItem` method

[bsunderhus]

Previous Behavior

Currently the only ways to get a headless tree item from the headless tree are:

1. by generating all the items with .items() method

2. by simulating a navigation and then using .getNextNavigableItem() method  

   New Behavior

3. modifies the return signature of useHeadlessFlatTree to include .getItem method that allow trying to locate the headless tree item

Related Issue(s)

• Fixes https://github.com/microsoft/fluentui/issues/30985

[codesandbox-ci[bot]]

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[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
FluentProviderWithTheme	virtual-rerender	37	38	10	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	650	615	5000
Button	mount	301	301	5000
Field	mount	1089	1105	5000
FluentProvider	mount	708	693	5000
FluentProviderWithTheme	mount	83	83	10
FluentProviderWithTheme	virtual-rerender	37	38	10	Possible regression
FluentProviderWithTheme	virtual-rerender-with-unmount	79	70	10
MakeStyles	mount	883	837	50000
Persona	mount	1775	1725	5000
SpinButton	mount	1382	1376	5000
SwatchPicker	mount	1546	1579	5000

[fabricteam]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-components react-components: entire library	1.162 MB 279.076 kB	1.162 MB 279.091 kB	47 B 15 B

Unchanged fixtures

| Package & Exports | Size (minified/GZIP) | | ----------------- | -------------------: | | react-components
react-components: Button, FluentProvider & webLightTheme | `71.55 kB`
`20.584 kB` | | react-components
react-components: Accordion, Button, FluentProvider, Image, Menu, Popover | `221.825 kB`
`62.568 kB` | | react-components
react-components: FluentProvider & webLightTheme | `44.037 kB`
`14.418 kB` | | react-portal-compat
PortalCompatProvider | `8.39 kB`
`2.64 kB` |

_{🤖 This report was generated against e69f90d32c4b4f11dfab5e563d2475d0d3f8940a}