Support running multiple handlers side-by-side in a custom server

[Janpot]

• Change the meaning of Toolpad root to actually point to the ./toolpad project folder. That way we can allow you to use multiple within the same folder
• We replace the default of . to ./toolpad to counter the removal of a path segment and avoid this being a breaking change
• Remove the deprecation message of legacy projects. We can't detect this anymore. And it's not in use anymore for a long time
• Relax the guard that allows to create only a single project instance to a per folder guard so multiple projects can run side by side in the same node.js process.
• read .env file from cwd
• introduce the --create option for CLI only as a protection against unintended project initialization