fix(VStepper): make step items clickable when non-linear prop is set

[sridhar-3009]

Fixes #19275

Root cause

VStepper provides defaults to VStepperItem (editable, icons, etc.) but never provided nonLinear. As a result, in VStepperItem, isClickable was computed as !props.disabled && props.editable — meaning the nonLinear prop had no effect on step navigation.

Fix

• In VStepper.tsx: include nonLinear in the provideDefaults for VStepperItem
• In VStepperItem.tsx: add nonLinear prop and update isClickable to !disabled && (editable || nonLinear)
• The disabled attribute and ripple on the button now use isClickable so non-linear steppers properly enable clicking without requiring editable

editable and nonLinear remain independent: editable also shows the edit icon on the active step, nonLinear only enables click navigation.

[J-Sek]

behavior might be revised in v5, but in v4 we only need docs change as described here