detect anvil, default PK and prepopulate .env in dev

[frolic]

this sort of error is annoying for first time users and hard to copy inside mprocs, so we could automate this away

[MUDError]: Missing PRIVATE_KEY environment variable.
Run 'echo "PRIVATE_KEY=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80" > .env'