Add syntax highlighting for GraphQL (develop an open-source grammar for it)

Created by: slimsag

The syntax definition we used for highlighting GraphQL files before was found to have no license and not be open-source so we had to remove it. See https://github.com/sourcegraph/sourcegraph/issues/13933

We should develop and open-source grammar for it following what was said in #13933 (closed) - please see that issue if you are interested in taking a shot at this. We are looking for volunteers :)