slot: Add customizable mergeProps

[chaance]

This PR adds entrypoints to control how Slot merges props with its child. You can now:

1. pass a mergeProps function to an individual Slot for one-off needs, or
2. Use the new Slot.Provider to apply a custom merge strategy to all nested components that use Slot under the hood.

import { Slot } from "radix-ui";

const mergeProps: Slot.MergePropsFunction = (slotProps, childProps) => {
  // your custom merge strategy, optionally delegating to the default
  const merged = Slot.mergeProps(slotProps, childProps);
  return {
    ...merged,
    className: classNameProcessor(slotProps, childProps),
    'data-custom-merge': 'true',
  };
};

// one-off
<Slot.Root mergeProps={mergeProps}>{child}</Slot.Root>

// applies to all nested `asChild` components
<Slot.Provider mergeProps={mergeProps}>
  <App />
</Slot.Provider>

Closes #2631

[changeset-bot]

🦋 Changeset detected

Latest commit: cabda16

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 44 packages

Name	Type
@radix-ui/react-slot	Minor
radix-ui	Minor
@radix-ui/react-collection	Patch
@radix-ui/react-dialog	Patch
@radix-ui/react-menu	Patch
@radix-ui/react-popover	Patch
@radix-ui/react-primitive	Patch
@radix-ui/react-select	Patch
@radix-ui/react-tooltip	Patch
@radix-ui/react-accordion	Patch
@radix-ui/react-menubar	Patch
@radix-ui/react-navigation-menu	Patch
@radix-ui/react-one-time-password-field	Patch
@radix-ui/react-roving-focus	Patch
@radix-ui/react-slider	Patch
@radix-ui/react-toast	Patch
@radix-ui/react-alert-dialog	Patch
@radix-ui/react-context-menu	Patch
@radix-ui/react-dropdown-menu	Patch
@radix-ui/react-announce	Patch
@radix-ui/react-arrow	Patch
@radix-ui/react-aspect-ratio	Patch
@radix-ui/react-avatar	Patch
@radix-ui/react-checkbox	Patch
@radix-ui/react-collapsible	Patch
@radix-ui/react-dismissable-layer	Patch
@radix-ui/react-focus-scope	Patch
@radix-ui/react-form	Patch
@radix-ui/react-hover-card	Patch
@radix-ui/react-label	Patch
@radix-ui/react-password-toggle-field	Patch
@radix-ui/react-popper	Patch
@radix-ui/react-portal	Patch
@radix-ui/react-progress	Patch
@radix-ui/react-radio-group	Patch
@radix-ui/react-scroll-area	Patch
@radix-ui/react-separator	Patch
@radix-ui/react-switch	Patch
@radix-ui/react-tabs	Patch
@radix-ui/react-toggle-group	Patch
@radix-ui/react-toggle	Patch
@radix-ui/react-toolbar	Patch
@radix-ui/react-visually-hidden	Patch
@radix-ui/react-accessible-icon	Patch

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