refactor(stepper): Stepper and step refactor

[rkaraivanov]

Description

• Sync stepper and steps through Lit context
• Abstracted state logic to a separate file
• Moved common types and animations to a common folder
• Updated tests and stories to reflect the changes

Type of Change

• Refactoring (code improvements without functional changes)

Testing

The test suite is rewritten to accommodate for the removed "public private" properties maintaining
the scenario and expected behavior of each unit test as close as possible.

Accessibility has been tested with the latest version on NVDA (2025.3.3) in Chrome and Firefox and
it is correctly announcing state and intention.

Checklist

• My code follows the project's coding standards
• I have tested my changes locally
• I have updated documentation if needed

[RivaIvanova]

Keyboard navigation with ArrowLeft and ArrowRight is working strangely when the stepper is vertical. It is not clear from the spec what the expected behavior exactly is, but pressing ArrowLeft always navigates to the next step and ArrowRight to the previous step. Even though the direction is LTR, it behaves as if the direction is RTL.

[rkaraivanov]

@RivaIvanova
Nice catch, I've restored the old behavior and will revise the specification.