Add .gitconfig file type to .editorconfig

The `.gitmodules` file defines the properties of a repository's submodules. The file automatically generated by Git
submodule commands use tabs for indentation.

It uses the same file format as the Git configuration file (e.g., `.gitconfig`). Even though the `.gitconfig` file is not
likely to be found under the repository tree, it's possible the `.editorconfig` might end up being used outside the
project specific scope so I added it to the file pattern.

.editorconfig

@@ -56,5 +56,5 @@ indent_style = space
 indent_size = 2
 indent_style = space
 
-[.gitmodules]
+[{.gitconfig,.gitmodules}]
 indent_style = tab