feat: Treat 'run' as default command

[popzxc]

What 💻

Makes era_test_node treat run as the default subcommand.

Why ✋

Ergonomics and L1 compatibility. You can run anvil without any arguments, I'd expect era_test_node to do the same.

Evidence 📷

[MexicanAce]

The key difference with anvil and era-test-node is that anvil is only a Node. Where as era-test-node is also a binary to help replay transactions and help users diagnose what could've gone wrong in their code.

By default, we chose to display the help screen to better highlight that this can be more of a tool than just another node.

That being said, it sounds like there's an expectation for default functionality to first-and-foremost match anvil, so updating the default (shouldn't be) a harmful, breaking change.

[MexicanAce]

LGTM