Add StringStream definition for codemirror

taken from https://codemirror.net/doc/manual.html#modeapi

Eventually we'll need to add in the actual mode object but that's a whole different beast.
I couldn't find documentation on lastColumnPos, lastColumnValue, or lineStart. Let me know if it's alright to just leave them undocumented. Thanks!
Merge pull request #2906 from hellochar/patch-2

Add StringStream definition for codemirror