Validate that the operation's root type is valid (#225)

Turns out neither we (nor anyone else) were validating this, because the
spec didn't say explicitly to validate it.  So you could do
`mutation { bogus }` even if the schema has no mutation types, or worse,
any syntactically valid query if the schema is totally empty.

In graphql/graphql-spec#955 I'm adding it to the spec, and in
graphql/graphql-js#3592 someone else is adding it to graphql-js.  So we
should too, once those land!  At that point we should also likely
reimport the relevant tests, instead of the ones I wrote here, but I
figured I'd put the PR up now so folks can review the non-test parts if
you like.

Fixes #221.

validator/imported/spec/schemas.yml

@@ -487,3 +487,4 @@
   }
 
   scalar Any
+- ""

validator/rules/known_root_type.go

@@ -0,0 +1,35 @@
+package validator
+
+import (
+	"fmt"
+
+	"github.com/vektah/gqlparser/v2/ast"
+	. "github.com/vektah/gqlparser/v2/validator"
+)
+
+func init() {
+	AddRule("KnownRootType", func(observers *Events, addError AddErrFunc) {
+		// A query's root must be a valid type.  Surprisingly, this isn't
+		// checked anywhere else!
+		observers.OnOperation(func(walker *Walker, operation *ast.OperationDefinition) {
+			var def *ast.Definition
+			switch operation.Operation {
+			case ast.Query, "":
+				def = walker.Schema.Query
+			case ast.Mutation:
+				def = walker.Schema.Mutation
+			case ast.Subscription:
+				def = walker.Schema.Subscription
+			default:
+				// This shouldn't even parse; if it did we probably need to
+				// update this switch block to add the new operation type.
+				panic(fmt.Sprintf(`got unknown operation type "%s"`, operation.Operation))
+			}
+			if def == nil {
+				addError(
+					Message(`Schema does not support operation type "%s"`, operation.Operation),
+					At(operation.Position))
+			}
+		})
+	})
+}

validator/spec/KnownRootTypeRule.spec.yml

@@ -0,0 +1,19 @@
+- name: Known root type
+  rule: KnownRootType
+  schema: 0
+  query: |
+    query { dog { name } }
+- name: Valid root type but not in schema
+  rule: KnownRootType
+  schema: 0
+  query: |
+    mutation { dog { name } }
+  errors:
+    - message: Schema does not support operation type "mutation"
+- name: Valid root type but schema is entirely empty
+  rule: KnownRootType
+  schema: 20
+  query: |
+    { dog { name } }
+  errors:
+    - message: Schema does not support operation type "query"