feat: add footer skip button

[milkatx]

Skippable Tour Branch

This branch contains an opt-in footer skip button for tours.

• Adds showButtons: ["skip", "previous", "next"]
• Adds skipBtnText for configuring the skip label
• Adds onSkipClick for custom skip behavior
• Keeps forced tours intact by hiding skip when allowClose: false
• Documents the new option and adds a local demo example

StackBlitz

https://stackblitz.com/github/milkatx/driver.js/tree/feat/footer-skip-button?file=index.html