[Stepper] Add Alternative Label on horizontal-type Stepper (#91496)

* Add label on Step

* Add _buildLabelText, _labelStyle

* Update _buildHorizontal with _buildLabelText

* Fix Redundant Center widgets

* Fix Unnecessary Column Widget

* Fix Unnecessary import

* Fix Unnecessary Container

* Set label style to bodyText1 to match title style

* Update test for Alternative Label

* Fix code format

* Update label docstring

* Update to test textstyle of label

* Update to Test `TextStyles` of Label

* Fix typo in inline comments

* Update for Stepper type to `horizontal`

* Restore `_buildVerticalHeader`

* Update docstring, `Only [StepperType.horizontal]`

* Update for Readability

* Add `_isLabel` method

* Update `Horizontal height` with `_isLabel`

* Update to make `_buildIcon` centered

* Fix for `curly_braces_in_flow_control_structures`