Disabling autocomplete in string and comment contexts

Fix for MAGN-5099: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5099

This refers to issue: https://github.com/DynamoDS/Dynamo/issues/2942

@Benglin please review
Yes you're right but this would be a syntax error and autocompletion is not supported in syntax errors currently.