refactor: import widgets.dart instead of material.dart

[LinXunFeng]

Related Issues

• Closes [Feature request] Flutter version 3.47 has been released, and the UI has been separated. This plugin needs to be adapted. #156

Description

Flutter 3.47 ships Material and Cupertino as the standalone material_ui and cupertino_ui packages, and the in-framework package:flutter/material.dart is scheduled for deprecation.

scrollview_observer doesn't use any Material widgets. lib/ only imported package:flutter/material.dart for the widgets, rendering and foundation symbols it re-exports. This PR replaces those imports with package:flutter/widgets.dart in all 27 files under lib/, so the package no longer depends on either design library.

• No material_ui dependency is added, so it works the same whether the app uses package:flutter/material.dart or package:material_ui.
• No Material types appear in the public API, so there are no type mismatches for apps that have migrated to material_ui.
• The SDK constraints (sdk: ">=2.19.0 <4.0.0", flutter: '>=3.7.0') stay unchanged, and there are no public API changes, so this ships as a patch release (1.27.3).
• The doc comment [SliverAppBar] in ObserverUtils.calcPersistentHeaderExtent becomes `SliverAppBar`, since that symbol is no longer in scope and would otherwise produce an unresolved dartdoc reference.

example/ and test/ still import package:flutter/material.dart. They are not part of the published library and will be migrated separately.

Tests

• Flutter 3.47.1: flutter test - 86 passed, flutter analyze . - no issues, dart format --set-exit-if-changed . - 0 changed
• Flutter 3.44.4 / 3.41.7: flutter analyze lib - no issues, flutter test - 86 passed
• Flutter 3.7.0: flutter analyze lib - no issues, sliver_observer_test.dart and observer_utils_test.dart passed
• Build a minimal app with Flutter 3.47.1 that only depends on material_ui (migrated with dart fix --apply --code=migrate_design_widgets), and verify ListViewObserver + jumpTo works via a widget test
• pana - 160/160