[Slider] Add useRelativeDrag prop for relative drag mode

[adids1221]

Description

Adds a new useRelativeDrag prop (default false).
When enabled, dragging anywhere on the slider moves the thumb relative to its current position instead of snapping to the touch point. Designed for single-thumb mode.

Depends on #3951.

Changelog

Slider - Added useRelativeDrag prop for relative drag mode where touch anywhere moves the thumb proportionally to drag distance.

Additional info

MADS-4866

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[adids1221]

How to reproduce

Drop this into PlaygroundScreen.tsx:

import React, {useState} from 'react';
import {Platform} from 'react-native';
import {View, Text, Slider, Switch} from 'react-native-ui-lib';

const TAG = `[Slider-${Platform.OS}]`;

export default function PlaygroundScreen() {
  const [disabled, setDisabled] = useState(false);
  const [useRelativeDrag, setUseRelativeDrag] = useState(false);

  return (
    <View flex padding-20>
      <Text text60 marginB-20>
        Slider Debug ({Platform.OS})
      </Text>

      <View row centerV marginB-10>
        <Text text70 marginR-10>Disabled</Text>
        <Switch value={disabled} onValueChange={setDisabled}/>
      </View>

      <View row centerV marginB-20>
        <Text text70 marginR-10>useRelativeDrag</Text>
        <Switch value={useRelativeDrag} onValueChange={setUseRelativeDrag}/>
      </View>

      <Text text70 marginB-10>
        Slider (default)
      </Text>
      <Slider
        value={30}
        minimumValue={0}
        maximumValue={100}
        disabled={disabled}
        onSeekStart={() => console.log(`${TAG} - default - onSeekStart`)}
        onSeekEnd={() => console.log(`${TAG} - default - onSeekEnd`)}
        onValueChange={(v: number) => console.log(`${TAG} - default - value: ${v.toFixed(1)}`)}
      />

      <Text text70 marginT-40 marginB-10>
        Slider (useRelativeDrag={String(useRelativeDrag)})
      </Text>
      <Slider
        value={30}
        minimumValue={0}
        maximumValue={100}
        disabled={disabled}
        useRelativeDrag={useRelativeDrag}
        onSeekStart={() => console.log(`${TAG} - relative - onSeekStart`)}
        onSeekEnd={() => console.log(`${TAG} - relative - onSeekEnd`)}
        onValueChange={(v: number) => console.log(`${TAG} - relative - value: ${v.toFixed(1)}`)}
      />
    </View>
  );
}

What to test

• Default slider — drag the thumb, tap the track to jump — should work as before
• useRelativeDrag slider — touch anywhere and drag — thumb moves relative to its current position, no snap on touch
• Toggle useRelativeDrag — switch between modes, verify behavior changes