Consider removing tc39/test262 submodule and download lazily

[mattsse]

Due to how cargo handles submodules (or rather doesn't): rust-lang/cargo#4247

a fresh cargo build with boa as git dependency boa { git = "..."} will download all submodules and it takes some time to download https://github.com/tc39/test262

Perhaps it's possible to download the repo lazily instead, for example:

dtolnay/syn#183

[gakonst]

hi @HalidOdat - just flagging again...thank you for all the work for this crate. also I think doing a release would also fix this?

[HalidOdat]

I think doing a release would also fix this?

This would be a temporary fix though, I'll create a PR with a proper fix :)