Add support for WebAssemly (WASM).

[timbotimbo2]

This is a retry of #1028, which ran into an outdated github action workflow.

Description

This PR adds WebAssemly (Wasm) support to the web platform, which was requested here #989.

Since Flutter 3.24 it supports 2 build modes on web, default and WebAssembly.

WebAssembly doesn't support the dart:html library that is used in this plugin.
dart:html needs to be switched to package:web.

This web package also supports the classic default mode for web, which means this switch is not a breaking change.
However the lowest compatible web package requires Flutter 3.16 and Dart 3.2.

This PR raises the minimum version of flutter_unity_widget to Flutter 3.16 and Dart 3.2.

To actually run this plugin with WebAssembly, web needs to be resolved to version 0.5.x or 1.x.y.
I've set the dependency in pubspec.yaml to >=0.3.0 < 2.0.0.
This is to keep support for lower Flutter versions without WebAssembly, down to Flutter 3.16.

Just a small note:
In my testing, pointer_interceptor which is needed to stack UI on top of the webview, will only work in WebAssembly mode when the minimum dart sdk of your project is 2.17 or higher.

Type of Change

Most Flutter versions do not mark upgrading the minimum Flutter version as a breaking change.

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore