feat: add SplitButton component

[draggie]

Motivation

Implements the new Material Design 3 SplitButton component as part of the v6 component modernization work.

The component provides a primary leading action and a separate trailing action for contextual options. It reuses the modernized Button API direction
from #4928/#4943: explicit label/icon props, MD3 mode names (filled, tonal, elevated, outlined), derived ripple/state-layer colors,
extracted component tokens, size metrics, shape tokens, disabled/loading states, and separate accessibility/press handlers for each button segment.

This also adds the component to the public exports, documentation generation config, theme color docs data, and the example app. The example includes a
Menu-based playground, mode showcase, loading/disabled controls, and custom color/label styling.

Related issue

Closes #4986

Related to #4928 and #4943

Test plan

• yarn lint-no-fix
  • Passes with one existing unrelated warning in src/components/__tests__/TextInput.test.tsx.
• yarn typescript
• yarn test --watchman=false
• yarn docs build

Manual verification in the example app:

• Open the SplitButton example screen.
• In the playground, tap the leading Send action and trailing menu action separately.
• Verify the trailing menu opens below the SplitButton anchor and does not cover the button.
• Toggle Disabled and verify both press targets are disabled and styled correctly.
• Toggle Loading and verify the leading icon is replaced with a spinner.
• Review all modes: filled, tonal, elevated, and outlined.
• Review custom color and custom label style examples.