fix: Add conditional export to support --wasm build

[koji-1009]

Added if (dart.library.html) to src/web/channel_legacy.dart export.

With this change, src/web/channel_legacy.dart will only be referenced if dart:html is available, and builds with the --wasm option will succeed.

---

If we write the following migration code from IndexedDB to WasmDB, we need to do two imports, drift/wasm.dart and drift/web.dart.

import 'package:drift/drift.dart';
import 'package:drift/wasm.dart';
import 'package:drift/web.dart';

const _databaseName = 'nice_db';

LazyDatabase getDatabase() => LazyDatabase(() async {
      final result = await WasmDatabase.open(
        databaseName: _databaseName,
        sqlite3Uri: Uri.parse('sqlite3.wasm'),
        driftWorkerUri: Uri.parse('drift_worker.dart.js'),
        initializeDatabase: () async {
          final storage = await DriftWebStorage.indexedDbIfSupported(
            _databaseName,
          );
          await storage.open();

          final blob = await storage.restore();
          await storage.close();

          return blob;
        },
      );

      return result.resolvedExecutor;
    });

In a build targeting the web, the current code work. (flutter run -d chrome)
However, in builds with the --wasm option, code using dart:html is not available. (flutter run -d chrome --wasm)

$ flutter run -d chrome --wasm
Launching lib/main.dart on Chrome in debug mode...
Target dart2wasm failed: ProcessException: Process exited abnormally with exit code 64:
../../../../.pub-cache/hosted/pub.dev/drift-2.22.0/lib/src/web/channel_legacy.dart:1:8: Error: Dart library 'dart:html' is not available on this platform.
import 'dart:html';
       ^
Context: The unavailable library 'dart:html' is imported through these packages:

    main.dart => package:example => package:encrypt_file_loader => package:drift => dart:html

[simolus3]

Thanks! I think this is a good way to ease the migration, as we can continue to keep this setup until dart:html is removed for good.

[koji-1009]

The code migration support via extentsion on dynamic looks great. I was not aware of that!
Thanks for the review!