Command to scaffold a new task directory with required files

[mds1]

There is no easy way to create new tasks. Add a command just new-task to the root justfile that takes the following arguments:

1. L1 network short name. Allowed network names are currently eth, oeth, opsep, sep, and sep-dev-0. Unrecognized network names should error
2. L2 network short name. Allowed network names are all short names from the superchain registry. Unrecognized network names should error
3. Task name. This must be a hyphenated string such as fjord-gas-config or guardian-upgrade
4. Task kind. Single or nested are the only allowed values for now. In the future we can expand this to other tasks, like presigned pauses, or the fault proof recovery template tasks.

The command should:

• Create a folder called tasks/{arg1}/{arg2}-{x}-{arg3}, where arg1 and arg3 match the above arguments. arg2 matches the above argument but is omitted if the L2 network is OP Mainnet (this is the current convention). Lastly, x is the next incrementing number for that L2's task sequence. For example if mode-002-* is the most recent task for mode, the next task would have x as 003.
• Create README.md and VALIDATION.md templates with placeholders for the user to fill out
• Create a template input.json file
• Create the appropriate SignFromJson.s.sol or NestedSignFromJson.s.sol with the appropriate imports, functions that need to be implemented, etc
• A .env file (or equivalent if Remove hardcoded addresses #336 is done first)