need to add target to .gitignore for js project

[ashleygwilliams]

in rust projects, target is already added to the .gitignore, but in other projects we can't assume that.

[Gankra]

I'm not sure we ever edit .gitignore rn but it wouldn't be wild for us to, and at very least we should update the JS guide to say this. More generally along with #1335 and #870 we're just very loosegoosey about target dirs rn and need to do more passes on locking that down.