Enable some Wasm features during the build

[tomaka]

cc #350

This PR enables the bulk-memory and signext Wasm features.
They are supported by all engines listed on https://webassembly.org/roadmap/.
The minimum NodeJS version would be 12.5, which is very old.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       -1114 ┊ smoldot::executor::host::ReadyToRun::run_once::hf0068349ac6bacf4
        -746 ┊ smoldot_light::json_rpc_service::background::Background<TPlat>::handle_request::{{closure}}::hf803cfc2f8737078
        -279 ┊ smoldot::executor::runtime_host::Inner::run::hcae027d7fdcc47be
        -187 ┊ smoldot_light::Client<TPlat,TChain>::add_chain::h9c2b133789f59748
        -168 ┊ smoldot::json_rpc::methods::MethodCall::from_defs::h088d4799535dc1d7
        -149 ┊ smoldot_light::sync_service::standalone::start_standalone_chain::{{closure}}::h144f02e74c122810
        -147 ┊ smoldot::executor::read_only_runtime_host::Inner::run::hf577253fe2316ddc
        -143 ┊ core::array::<impl core::default::Default for [T; _]>::default::hb2bbea0216367016
        -140 ┊ smoldot::chain::chain_information::build::ChainInformationBuild::from_call_in_progress::h88fc5ef399993c7d
        -129 ┊ smoldot_light::sync_service::parachain::ParachainBackgroundTask<TPlat>::start_paraheads_fetch::{{closure}}::h23b235044e9dad69
        -122 ┊ smoldot::sync::warp_sync::BuildRuntime<TSrc,TRq>::build::he0781fbd0aa36bb8
        -111 ┊ smoldot::libp2p::connection::established::substream::Substream<TNow,TRqUd,TNotifUd>::read_write2::h170ef27f01dbf869
        -108 ┊ <core::panic::unwind_safe::AssertUnwindSafe<F> as core::future::future::Future>::poll::h96117b949f01bebc
        -108 ┊ smoldot::json_rpc::requests_subscriptions::SubscriptionStart<TSubMsg>::start::{{closure}}::hd1869a5bdbd2f83b
        -105 ┊ smoldot_light::network_service::tasks::connection_task::{{closure}}::he90ae6bf8d9fc3ff
         -96 ┊ libsecp256k1_core::group::Jacobian::add_ge_var_in_place::h047ccad52ee3decb
         -94 ┊ smoldot_light::transactions_service::background_task::{{closure}}::h81cac1af61aedd15
         -90 ┊ smoldot::sync::all::AllSync<TRq,TSrc,TBl>::process_one::hb712fb39ab470346
         -89 ┊ libsecp256k1_core::group::Jacobian::add_zinv_var_in_place::hd95b4caeb558d313
         -87 ┊ core::array::<impl core::default::Default for [T; _]>::default::h50766cd2abeef611
      -15304 ┊ ... and 2148 more.
      -19612 ┊ Σ [2168 Total Rows]