Disabling autocomplete in string and comment contexts

Fix for MAGN-5099: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5099

Will add test cases shortly.

@Benglin please review
Is it because of the name `IsInsideCommentOrString` that I am confused about its usage? Because it is supposed to tell me *okay, you're on line 3 and character 5, you're in a comment*. Something along the line of:

```csharp
var yes = IsInsideCommentOrString(text, 3, 5);
```

About the overall parser, perhaps the IDE parser does not have to be ultra complete but it can be complete in terms of its grammatical accuracy, it just needs to be more forgiving. At least given things like:

```csharp
// The following does not end with semi-colon...
var = { /* first entry */ 1, 2, /* last entry */ 3 }
```

It should still be able to tell you that ```Line 2 Col 16``` is within a comment. Maybe a user story for this improvement in near future? You want this to be perfect don't you? ;)