[web/wasm] Update Flutter Wasm browser compatibility matrix, deferred loading, --no-deprecated-js-interop, and FlutterConfiguration reference for 3.50 #13961

Expected fix

1. sites/docs/src/content/platform-integration/web/wasm.md (https://docs.flutter.dev/platform-integration/web/wasm)

• Browser compatibility matrix (L149-L180):
  • Explain that Flutter 3.50 Wasm (dart2wasm + skwasm) requires three standardized WebAssembly features: WasmGC, JS String Builtins (wasm:js-string), and Phase 4 Exception Handling (try_table / exnref) (flutter/flutter#193090, flutter/flutter#193271).
  • Remove the outdated Firefox 120 / Safari bug bullets and the warning claiming Flutter Wasm cannot run on iOS browsers (L163-L180).
  • Document the Flutter 3.50 browser matrix and fallback behavior for dual builds (flutter build web --wasm):

• Deferred loading note (L73-L84):
  • Update "This is available on the flutter main branch and will become generally available later this year in the 3.50 flutter stable release" to state that --enable-wasm-deferred-loading is available in Flutter 3.50.
• JS interop & diagnostics (L207-L259):
  • Document --[no-]deprecated-js-interop (flutter/flutter#193877) on flutter build web, flutter run, flutter drive, and flutter test --platform chrome, which turns legacy dart:html, dart:html_common, dart:indexed_db, dart:js, dart:js_util, dart:svg, dart:web_audio, dart:web_gl, and package:js imports into compile-time errors on JavaScript (dart2js / ddc) compilations (including the dart2js fallback build produced by flutter build web --wasm), sets dart.library.html etc. to false in conditional imports, and suppresses redundant Wasm dry-run console warnings (dart2wasm itself unconditionally disallows these libraries).
  • Remove the outdated "Ignore the long stack trace and command string" workaround on L232-L237 (flutter_tools now prints only the clean import path summary after flutter/flutter#190476).

2. sites/docs/src/content/platform-integration/web/initialization.md (https://docs.flutter.dev/platform-integration/web/initialization)

• Add the missing FlutterConfiguration properties (from engine/src/flutter/lib/web_ui/flutter_js/src/types.d.ts) to the config table (L118-L133):

• Add a short example under ## Example usage showing how to opt Safari 26.2+ and Firefox 147+ into Wasm via wasmAllowList:

_flutter.loader.load({
  config: {
    wasmAllowList: {
      blink: true,
      webkit: true,
      gecko: true,
    },
  },
});