feat: add `steps` to `melos.yaml` schema

[KoheiKanagu]

Supported https://melos.invertase.dev/configuration/scripts#steps

[aroemen]

@blaugold can this be merged to support steps?

[BoyleSeo]

@blaugold This feature is very important for developing on Windows systems. See this issue.

Until this PR is merged, as a Windows user I have to manually run the script by concatenating all command lines with &&...