[ruby/optparse] Add EditorConfig file

More info here: https://editorconfig.org/

For example, `ruby/ruby` has it: https://github.com/ruby/ruby/blob/05ebaee/.editorconfig

Also fix some offenses.

https://github.com/ruby/optparse/commit/29402e7e0e

test/optparse/test_acceptable.rb

@@ -196,4 +196,3 @@ class TestOptionParser::Acceptable < TestOptionParser
   end
 
 end
-