v1.5.1 of showcase_tutorial

A Flutter package to Showcase/Highlight widgets step by step. Changelog excerpt: - DOCS: add a second example page ("Feature demos"), reachable from a button on the original mail demo, that walks through the newer features (left/right tooltip positions, progress indicator, multi-widget highlight, custom action text, highlightExactShape, onShow/onDismiss, barrierInteraction, auto-skip). Add a feature-walkthrough GIF to the README preview and complete the README's feature list. No library changes.