Fix --dev flag for launching local developer node

[gztensor]

Substrate node template has a --dev flag, with which a special development network with a single node is running. It's very convenient for development as instead of running scripts and multiple nodes, you just use cargo run -- --dev.

To add this flag we need to specify a new chain spec. The same as for localnet, but with just alice authority would work. Also, we need to extend the CLI with this flag.