Fix DS.Model.belongsTo and DS.Model.hasMany args (#25072)

types/ember-data/index.d.ts

@@ -521,11 +521,11 @@ declare module 'ember-data' {
             /**
              * Get the reference for the specified belongsTo relationship.
              */
-            belongsTo(name: keyof ModelRegistry): BelongsToReference;
+            belongsTo(name: RelationshipsFor<this>): BelongsToReference;
             /**
              * Get the reference for the specified hasMany relationship.
              */
-            hasMany(name: keyof ModelRegistry): HasManyReference<any>;
+            hasMany(name: RelationshipsFor<this>): HasManyReference<any>;
             /**
              * Given a callback, iterates over each of the relationships in the model,
              * invoking the callback with the name of each relationship and its relationship

types/ember-data/test/relationships.ts

@@ -16,17 +16,23 @@ class Comment extends DS.Model {
     author = DS.attr('string');
 }
 
+class Series extends DS.Model {
+    title = DS.attr('string');
+}
+
 class RelationalPost extends DS.Model {
     title = DS.attr('string');
     tag = DS.attr('string');
     comments = DS.hasMany('comment', { async: true });
     relatedPosts = DS.hasMany('post');
+    series = DS.belongsTo('series');
 }
 
 declare module 'ember-data' {
     interface ModelRegistry {
         'relational-post': RelationalPost;
         comment: Comment;
+        series: Series;
     }
 }
 
@@ -35,3 +41,6 @@ blogPost!.get('comments').then((comments) => {
     // now we can work with the comments
     let author: string = comments.get('firstObject')!.get('author');
 });
+
+blogPost!.hasMany('relatedPosts');
+blogPost!.belongsTo('series');