rename workers-assets-gen command to workers-cli and add init command

[syumai]

• rename current workers-assets-gen command to workers-cli.
  • leaving alias of workers-assets-gen is not necessaly, because its executed by go run command with its specific version.
    • https://github.com/syumai/workers/blob/v0.26.0/_templates/cloudflare/worker-go/Makefile#L7
  • but some developers may rewrite its version in Makefile directly.
  • therefore, an alias with warning message should be kept.
• split generate command for asset generation.
• add init command for initialize current Go application as Cloudflare Workers' application.