fix: JSON block string encoding

[mattjohnsonpint]

Fixes #839

Note, I considered using gjson.AppendJSONString for better performance, but it currently doesn't have an option to disable escaping of HTML characters - which I think would be undesirable here. See tidwall/gjson#362

[mattjohnsonpint]

And now, after checking the relevant part of the GraphQL spec, I also realize that block quote indentation is not being handled correctly.

For example:

query {
  foo(bar: 
"""
abc
def
""")
}

and

query {
  foo(bar: 
      """
      abc
      def
      """)
}

... should be treated equivalently. Currently, the latter includes extraneous whitespace on the second line.

Update forthcoming...

[mattjohnsonpint]

Block indentation has been fixed, with passing tests.

BTW, I tried figuring out how to fix it in the lexer, but ran into problems because the resulting tokens are always just start/end pointers back to the original raw text. For the indentation parsing algorithm to work correctly, it has to be able to exclude data from the middle of a character sequence. So instead, I just wrote functions to find the original raw block string, and then parsed it according to the spec. Hope that works for y'all.

[StarpTech]

@mattjohnsonpint thank you!