build: fix type generation script (#867)

Due to the recent changes to the `tsconfig.json`, type imports need to be marked as such, so that the compiler knows to strip it out from the output.
By marking the type import, the script now runs correctly

scripts/generate-types.ts

@@ -2,7 +2,7 @@
 
 import { strict as assert } from "assert";
 import * as fs from "fs";
-import { JSONSchema7, JSONSchema7Definition } from "json-schema";
+import type { JSONSchema7, JSONSchema7Definition } from "json-schema";
 import { format } from "prettier";
 
 type JSONSchemaWithRef = JSONSchema7 & Required<Pick<JSONSchema7, "$ref">>;