sorbet: Add a RBI file for utils/svn.rb typechecking

- Needed to `include Kernel` otherwise Sorbet couldn't find the
  `system_command` method.
- I've also attempted to construct method signatures for all the methods.
- Before, `brew typecheck` surfaced 36 errors. Now we're down to 33 errors.

Library/Homebrew/sorbet/rbi/utils/svn.rbi

@@ -0,0 +1,14 @@
+# typed: strict
+
+module Utils::Svn
+  include Kernel
+
+  sig { returns(T::Boolean) }
+  def available?; end
+
+  sig { returns(T.nilable(String)) }
+  def version; end
+
+  sig { params(url: String).returns(T::Boolean) }
+  def remote_exists?(url); end
+end