fix(core): use bigint for int64 types (#829)

There are two types of integers supported in OpenAPI spec: `int32` and
`int64`. The current version of `orval`, however, converts all those
integers to js `number` and doesn't consider `int64` types as `bigint`.

Check the type of the integer and use `bigint` instead of `number` if
it is an `int64`.

packages/core/src/getters/scalar.ts

@@ -28,7 +28,7 @@ export const getScalar = ({
   switch (item.type) {
     case 'number':
     case 'integer': {
-      let value = 'number';
+      let value = item.format === 'int64' ? 'bigint' : 'number';
       let isEnum = false;
 
       if (item.enum) {