Description

SchemaExtension nodes are valid graphql but prettier doesn't currently know how to handle them. This adds the ability to print these nodes.

Worth mentioning:

• SchemaExtension doesn't support description the way SchemaDefinition does
• node.operationTypes is optional on SchemaExtension whereas it's required on a SchemaDefinition
• When node.operationTypes is empty, no { } should be printed

Relevant bits in the language spec: https://spec.graphql.org/October2021/#sec-Schema-Extension

Checklist

• I’ve added tests to confirm my change works.
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I’ve read the contributing guidelines.

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