Use tmLanguage file from MarkdownEditing package instead.

The MarkdownEditing package disables the default Markdown package, causing an error to appear and syntax highlighting to not work. I've changed the file to reference the tmLanguage from MarkdownEditing instead which works around this problem.

A more robust solution would be able to detect if MarkdownEditing is installed and automatically use the correct version. I haven't yet looked into how to do that.