fix: Support for ruby3.2

Dir.exists? was removed in Ruby 3.2.(https://docs.ruby-lang.org/ja/3.0/method/Dir/s/exists=3f.html)
Changed to use Dir.exist?

Related Issues
SciRuby#636

ext/nmatrix/extconf.rb

@@ -54,13 +54,13 @@
 create_conf_h("nmatrix_config.h")
 create_makefile("nmatrix")
 
-Dir.mkdir("data") unless Dir.exists?("data")
-Dir.mkdir("util") unless Dir.exists?("util")
-Dir.mkdir("storage") unless Dir.exists?("storage")
+Dir.mkdir("data") unless Dir.exist?("data")
+Dir.mkdir("util") unless Dir.exist?("util")
+Dir.mkdir("storage") unless Dir.exist?("storage")
 Dir.chdir("storage") do
-  Dir.mkdir("yale")  unless Dir.exists?("yale")
-  Dir.mkdir("list")  unless Dir.exists?("list")
-  Dir.mkdir("dense") unless Dir.exists?("dense")
+  Dir.mkdir("yale")  unless Dir.exist?("yale")
+  Dir.mkdir("list")  unless Dir.exist?("list")
+  Dir.mkdir("dense") unless Dir.exist?("dense")
 end
 
 # to clean up object files in subdirectories: