feat: add Steepfile with the minimum configuration

This file indicates that type definition files are placed in the `sig` directory and the actual implementation files that I want to define types for are located in the `lib` directory.
Additionally, it is configured to disable type error checking so as not to raise much errors.

Steepfile

@@ -0,0 +1,8 @@
+D = Steep::Diagnostic
+
+target :lib do
+  signature "sig"
+  check "lib" # Directory name to check
+
+  configure_code_diagnostics(D::Ruby.silent) # `silent` diagnostics setting
+end