Flexible diagnostics configuration

[soutaro]

This PR adds a new diagnostics configuration via #configure_code_diagnostics method.

It yields a hash, and you can update the hash in the block. Or you can pass a hash to #merge! the configuration.

D = Steep::Diagnostic

target :app do
  configure_code_diagnostics do |hash|
    hash[D::Ruby::UnexpectedPositionalArgument] = :error
  end
end

target :test do
  configure_code_diagnostics D::Ruby.lenient
end

• Implement new diagnostics configuration mechanics
• Add default configurations
• Fix output tests
• Add option to CLI to configure the threshold to be reported as an error
• Fix Steepfile template