BlockStringCharacter production doesn't quite work

The definition of BlockStringCharacter is:

```
BlockStringCharacter ::
- SourceCharacter but not `"""` or `\"""`
- `\"""`
```

But if I understand correctly, SourceCharacter is always a single character. So the concept of `but not`-ing a multi-character sequence out of SourceCharacter doesn't really make sense.

I think this should be something more like

```
BlockStringCharacter ::
- SourceCharacter but not `"` or `\`
- `"` [lookahead != `"`]
- `""` [lookahead != `"`]
- `\` [lookahead != `"`]
- `\"` [lookahead != `"`]
- `\""` [lookahead != `"`]
- `\"""`
```

I'm also not super clear after reading the spec if, eg,

```
"""

\""""

"""
```

is legit. After all you could interpret the spec as saying "three double quotes cannot be in the block string unless there's a backslash immediately before them" which is not the case here. (My version would make that allowed and parsed as `\"""` followed by `"`. graphql-js certainly treats it this way.)

Thoughts?