fix: support discriminators used with oneOf (#704)

* fix: support discriminators used with oneOf

* fix: seperate out the nested discriminator rule

Author: tinygrasshopper

src/rulesets/rest/2022-05-25/__tests__/__snapshots__/end-end.test.ts.snap

@@ -107,7 +107,31 @@ exports[`end-end-tests fails for the right reason with invalid parameters 1`] =
     "expected": undefined,
     "isMust": true,
     "isShould": false,
-    "name": "no mapping objects in discriminators",
+    "name": "discriminator usage rules",
+    "passed": true,
+    "received": undefined,
+    "severity": 2,
+    "sourcemap": {
+      "endLine": 362,
+      "endPosition": 12008,
+      "filePath": "/end-end-tests/api-standards/resources/repos/2022-04-04/spec.yaml",
+      "preview": "",
+      "startLine": 1,
+      "startPosition": 0,
+    },
+    "type": "changed",
+    "where": "specification",
+  },
+  {
+    "change": null,
+    "condition": undefined,
+    "docsLink": "https://github.com/snyk/sweater-comb/blob/main/docs/standards/rest.md#polymorphic-objects",
+    "error": undefined,
+    "exempted": false,
+    "expected": undefined,
+    "isMust": true,
+    "isShould": false,
+    "name": "no nested discriminators",
     "passed": true,
     "received": undefined,
     "severity": 2,
@@ -166750,7 +166774,31 @@ exports[`end-end-tests passes valid bulk POST operation 1`] = `
     "expected": undefined,
     "isMust": true,
     "isShould": false,
-    "name": "no mapping objects in discriminators",
+    "name": "discriminator usage rules",
+    "passed": true,
+    "received": undefined,
+    "severity": 2,
+    "sourcemap": {
+      "endLine": 120,
+      "endPosition": 3986,
+      "filePath": "/end-end-tests/api-standards/resources/thing/2021-11-10/000-batch-post.yaml",
+      "preview": "",
+      "startLine": 1,
+      "startPosition": 0,
+    },
+    "type": "changed",
+    "where": "specification",
+  },
+  {
+    "change": null,
+    "condition": undefined,
+    "docsLink": "https://github.com/snyk/sweater-comb/blob/main/docs/standards/rest.md#polymorphic-objects",
+    "error": undefined,
+    "exempted": false,
+    "expected": undefined,
+    "isMust": true,
+    "isShould": false,
+    "name": "no nested discriminators",
     "passed": true,
     "received": undefined,
     "severity": 2,