feat(core/graphcache): Add support for client-side-only directive processing

[kitten]

Summary

When applied, this change alters formatDocument to return a FormattedNode type mapping. This maps GraphQL AST nodes and specifically:

• adds a _generated: true flag on added __typename fields so Graphcache can identify them as relevant only to introspection
• adds a _directives: {...} dictionary to selection nodes that contains a mapping of directive names to directive nodes for client-side directives
• filters directives to not include client-side directives, i.e. directive starting with "_" underscores

This will allow us to implement client-side directives in Graphcache as well as stabilise formatDocument to do more advanced processing of GraphQL ASTs.

Set of changes

• @urql/core
  • Move formatDocument to core/src/utils/formatDocument.ts
  • Rename internal collectTypesFromResponse to collectTypenames and move it to core/src/utils/collectTypenames.ts
  • Add FormattedNode type mapping for formatDocument
  • Add directive handling to formatDocument
  • Add _generated: true annotation to __typename fields that formatDocument adds
  • Remove hasChanged checks in formatDocument
• @urql/exchange-graphcache
  • Remove remapping of "original operations" and instead rely on skipping _generated: true in cache reads
  • Use _directives map instead of directives list for @include, @skip and @defer
  • Update to always require FormattedNode AST nodes throughout the codebase