Consider reusing forge-std's StdChains

[mds1]

Now that I think of it, we probably can reuse forge-std's StdChains to avoid having to maintain our own list of constants. And if there are chains forge-std is missing that we need, we can PR those into forge-std. Looks like forge-std doesn't have a way to just fetch chain ID from a chain alias but that's an easy helper method to upstream as well

Originally posted by @mds1 in #467 (comment)

[blmalone]

This is being implemented by @ElliotFriedman

[ElliotFriedman]

1. create upstream PR in foundry with the L2 chainids and RPC URL's we need
2. remove the chainids from the Constants file