fix: items must be a schema (and add prefixItems)

This was changed in JSON Schema Specification Draft 2020-12
(which is the version of JSON Schema used by OpenAPI 3.1).
https://json-schema.org/draft/2020-12/json-schema-core#name-changelog:

> Array-value "items" functionality is now "prefixItems"

Author: not-my-profile

src/schema.ts

@@ -75,7 +75,8 @@ type IntegerSchema = {
 
 type ArraySchema = {
   type: "array",
-  items: SchemaObject | SchemaObject[],
+  items?: SchemaObject,
+  prefixItems?: SchemaObject[],
   minItems?: number,
   maxItems?: number,
   uniqueItems?: boolean,