ast+topdown: Add graphql.schema_is_valid builtin. (#5296)

This commit adds a new GraphQL builtin for validating GraphQL schemas,
applying stronger validation rules than what the current GraphQL parsing
builtins apply by default.

Fixes: #5125

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>

capabilities.json

@@ -1288,6 +1288,36 @@
         "type": "function"
       }
     },
+    {
+      "name": "graphql.schema_is_valid",
+      "decl": {
+        "args": [
+          {
+            "of": [
+              {
+                "type": "string"
+              },
+              {
+                "dynamic": {
+                  "key": {
+                    "type": "any"
+                  },
+                  "value": {
+                    "type": "any"
+                  }
+                },
+                "type": "object"
+              }
+            ],
+            "type": "any"
+          }
+        ],
+        "result": {
+          "type": "boolean"
+        },
+        "type": "function"
+      }
+    },
     {
       "name": "gt",
       "decl": {