Add a first experimental version of <gr-revision-parents>

This is flag protected by `UiFeature__revision_parents_data` and only
visible to developers.

This change only sets up the new component and the code structure. It
surfaces the data for developer purposes only and looks like this:
https://imgur.com/a/v6dhdwn. The actual UI design will happen in
subsequent changes.

We are requesting the `PARENTS` change detail option, which is not even
flag protected. It will just return more information that we will
definitely need for this feature. And this is not expected to add any
substantial latency.

We have also refined the type model of `RevisionInfo` by removing
`basePatchNum` from it, which should only be a property of
`EditRevisionInfo`.

Release-Notes: skip
Google-Bug-Id: b/289177174
Change-Id: I271d8cdcc88de68d2f7738a06b23909ef3e8f1c0
diff --git a/polygerrit-ui/app/types/types.ts b/polygerrit-ui/app/types/types.ts
index 9f2374a..c9f3414 100644
--- a/polygerrit-ui/app/types/types.ts
+++ b/polygerrit-ui/app/types/types.ts
@@ -116,8 +116,12 @@
   updates: {message: string; reviewers: AccountInfo[]}[];
 }
 
+/**
+ * https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#edit-info
+ */
 export interface EditRevisionInfo extends Partial<RevisionInfo> {
   // EditRevisionInfo has less required properties then RevisionInfo
+  // TODO: Explicitly list which props are required and optional here.
   _number: EditPatchSet;
   basePatchNum: BasePatchSetNum;
   commit: CommitInfo;