WIP: Add sembast_web and load correct databaseFactoryWeb

[nstrelow]

Work in progress: I would appreciate if this is used as a starting point to get sembast working on web.
Feel free to use these changes in your own PR to fix it 🚀

I added the necessary packages sembast_web and included the correct databaseFactory.

I have not really tested this, as we currently do not use Sembast.

Includes web fixes from #395.

sembast_web has certain limitations:

Codec are not supported. Web is not safe anyway. Encrypt fields as needed.

from core_store_sem_impl.dart:

final SembastCodec codec = getXXTeaSembastCodec(password: password);
final Database db = await factory.openDatabase(dbPath, codec: codec);

Fails with error:

Error: Expected a value of type 'Map<String, dynamic>', but got one of type 'String'
    at Object.throw_ [as throw] (http://localhost:53792/dart_sdk.js:4463:11)
    at Object.castError (http://localhost:53792/dart_sdk.js:4434:15)
    at Object.cast [as as] (http://localhost:53792/dart_sdk.js:4750:17)
    at dart.LegacyType.new.as (http://localhost:53792/dart_sdk.js:6274:60)
    at parse_server_sdk._XXTeaCodec.new.encode (http://localhost:53792/dart_sdk.js:43409:11)
    at jdb_factory_idb.JdbDatabaseIdb.new._txnAddEntries (http://localhost:53792/packages/sembast_web/src/jdb_factory_idb.dart.lib.js:355:49)
    at _txnAddEntries.next (<anonymous>)
    at http://localhost:53792/dart_sdk.js:37175:33
    at _RootZone.runUnary (http://localhost:53792/dart_sdk.js:37029:58)
    at _FutureListener.thenAwait.handleValue (http://localhost:53792/dart_sdk.js:32116:29)
    at handleValueCallback (http://localhost:53792/dart_sdk.js:32663:49)
    at Function._propagateToListeners (http://localhost:53792/dart_sdk.js:32701:17)
    at _Future.new.[_complete] (http://localhost:53792/dart_sdk.js:32535:25)
    at _SyncCompleter.new.complete (http://localhost:53792/dart_sdk.js:32043:31)
    at http://localhost:53792/dart_sdk.js:63461:17
    at Object._checkAndCall (http://localhost:53792/dart_sdk.js:4673:16)
    at Object.dcall (http://localhost:53792/dart_sdk.js:4678:17)
    at IDBRequest.<anonymous> (http://localhost:53792/dart_sdk.js:104624:21)Application finished.

[nstrelow]

We cannot use codec, which is needed for encryption. Would it still make sense to use sembast for it's other advantages?
Then we could simply remove the code/password/encryption part on web and add a disclaimer that sembast_web does not support it.

Or we encrypt/decrypt values before/after accessing sembast via a different encryption method.

[nstrelow]

Has been completed in #436