feat: build with reusable workflow

[guitarrapc]

tl;dr;

• Change CI to use reusable workflow.
  • update-packagejson: use Actions repository's workflow.
  • clean-packagejson-branch: use Actions repository's workflow.
• Change CI to use actions.
  • setup-dotnet: use Actions repository's action.
  • check-meta: use Actions repository's action.
• add <IsPackable>bool</IsPackable> to each csproj. This enable dotnet pack sln, no need to pack each csproj.

Note

• dotnet-build is simple step, so intended not to be reusable workflow.
• unity-build is also simple step, so intended not to be reusable workflow.