feat: Vega OS (Fire TV) support

[kalenkou]

Summary:

Adds Vega OS, Amazon's React Native platform on new Fire TV devices. Today the slider shows "Unimplemented component: RNCSlider" there, because Vega has no native slider view

The PR adds one source file, RNCSliderNativeComponent.kepler.tsx, with its tests. Metro picks it on Vega, like .web.tsx on the web. It draws the slider in React Native and works with a TV remote:

• It takes D-pad focus. With focus, Left and Right move the value by one step.
• A white ring on the thumb shows the focus.
• VoiceView (the Vega screen reader) reads it as an adjustable control.
• minimumValue, maximumValue, step, limits, inverted, disabled, tint colors and thumbImage work.
• Not yet on Vega (listed in the README): vertical, tapToSeek, track images, StepMarker.

Android, iOS, Windows and web are unchanged. The README gets a short "Vega OS (Fire TV)" section.

Test Plan:

• cd package && npm run lint && npm test: 31 tests pass, 15 of them new. npx tsc --noEmit passes.
• Fire TV Stick 4K Select (Vega OS 2.0): a test app with a button and <Slider minimumValue={0} maximumValue={100} step={1} value={50} />. Down moves the focus to the slider. Right, Right, Left give 51, 52, 51. Same result in three runs.

The same test app on the stick, before and after:

Context: I build Fire TV apps on Vega OS and hit this while porting libraries to it.