Fix query complexity for fragments defined after operations

[kasparsklavins]

Fixes #785, test shamelessly taken from #1350

QueryComplexity collects all field nodes and their definitions using a NodeKind::SELECTION_SET visitor. When NodeKind::OPERATION_DEFINITION visitor was run, complexity function, on the field's definition, for each found field was executed.

However, that's not enough when using fragments. When NodeKind::OPERATION_DEFINITION is run, field definitions for later-defined fragments have not yet been collected:

query {
  ...exampleFragment # <-- NodeKind::SELECTION_SET visitor collects definition for only `example`
} # <-- NodeKind::OPERATION_DEFINITION visitor

fragment exampleFragment on Query {
  example {
    a # not visited
    b # not visited 
  }
}

Complexity function for non-collected fields (a and b, in this case) was not run, and a fallback of value 1 was always used instead.

---

This PR changes QueryComplexity to use a NodeKind::DOCUMENT visitor in place of NodeKind::OPERATION_DEFINITION, as by then it's guaranteed all selection sets have been visited, and field definitions collected.

There is a minor behaviour change with this - when query complexity is exceeded, an error will now be thrown later during validation than before. As error order is not part of the api contract, I assume this is acceptable.

[spawnia]

Thank you, released with https://github.com/webonyx/graphql-php/releases/tag/v15.29.4.