feat: create FlatTreeItem component

[bsunderhus]

Previous Behavior

New Behavior

1. adds FlatTreeItem component
2. updates flat tree stories to use FlatTreeItem

[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
InfoButton	mount	13	13	5000	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	607	618	5000
Button	mount	310	309	5000
Field	mount	1108	1146	5000
FluentProvider	mount	697	709	5000
FluentProviderWithTheme	mount	75	76	10
FluentProviderWithTheme	virtual-rerender	74	59	10
FluentProviderWithTheme	virtual-rerender-with-unmount	76	69	10
InfoButton	mount	13	13	5000	Possible regression
MakeStyles	mount	867	849	50000
Persona	mount	1765	1720	5000
SpinButton	mount	1363	1343	5000

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 2f12533:

Sandbox	Source
@fluentui/react 8 starter	Configuration
@fluentui/react-components 9 starter	Configuration

[fabricteam]

📊 Bundle size report

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
react-components react-components: Button, FluentProvider & webLightTheme	69.517 kB 19.64 kB
react-components react-components: Accordion, Button, FluentProvider, Image, Menu, Popover	208.327 kB 59.386 kB
react-components react-components: FluentProvider & webLightTheme	40.713 kB 13.509 kB
react-portal-compat PortalCompatProvider	6.541 kB 2.227 kB

_{🤖 This report was generated against cbcc41cab55eb19e12faf363d0c96adb45ee2cb0}

[size-auditor]

Asset size changes

Size Auditor did not detect a change in bundle size for any component!

Baseline commit: cbcc41cab55eb19e12faf363d0c96adb45ee2cb0 (build)

[fabricteam]

🕵 fluentuiv9 No visual regressions between this PR and main