feat(react-switch): add useSwitchBase_unstable hook

[dmytrokirpa]

Summary

• Enables the public API for useSwitchBase_unstable hook in react-switch
• Exports useSwitchBase_unstable hook from the package index.ts
• Exports SwitchBaseProps and SwitchBaseState types from the package index.ts

The implementation was already present on the experimental branch (added in an earlier commit) but the exports were commented out with a note "Experimental APIs - will be uncommented in experimental branch". This PR enables those exports.

What the base hook includes

useSwitchBase_unstable manages:

• Slot structure (root, indicator, input, label slots)
• role="switch" ARIA attribute on input
• Checked/unchecked state (controlled and uncontrolled)
• labelPosition structural prop
• Focus management via useFocusWithin
• Field integration via useFieldControlProps_unstable
• onChange callback with typed data

useSwitchBase_unstable excludes:

• size prop (small/medium - design concern)

Test plan

• Verify useSwitchBase_unstable can be imported from @fluentui/react-switch
• Verify SwitchBaseProps and SwitchBaseState types are exported
• Verify existing useSwitch_unstable behavior is unchanged

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