[🔥AUDIT🔥] [rg.audit.3] Fix keys typing (#620)

🖍 _This is an audit!_ 🖍

## Summary:
This updates the typing of the `keys` function to match better with our expected usage.

Issue: FEI-4960

## Test plan:
`yarn typecheck`

Also built the types and tried them in webapp to verify they now work as we would want.

Author: somewhatabstract

Auditors: #typescript, #frontend-infra

Required Reviewers:

Approved By:

Checks: ⏭  dependabot, ⌛ gerald, ⌛ Prime node_modules cache for primary configuration (ubuntu-latest, 16.x), ⌛ Analyze (javascript)

Pull Request URL: #620

.changeset/stupid-beds-walk.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/wonder-stuff-core": patch
+---
+
+Fix the typing for the `key` function

packages/wonder-stuff-core/src/__tests__/keys.typestest.ts

@@ -19,10 +19,7 @@ import {keys} from "../keys";
         c: [3, 4],
     } as const;
 
-    // It would be nice if this worked, but TypeScript's library definition
-    // defines the return type of Object.keys() to be Array<string>.
     const keys2bad = keys(obj2);
-    // @ts-expect-error: string is not assignable to "a" | "b" | "c"
     const _: "a" | "b" | "c" = keys2bad[0];
 
     // @ts-expect-error: This errors because we try to get a key of only one type.

packages/wonder-stuff-core/src/keys.ts

@@ -1,12 +1,11 @@
 /**
  * Return an array of the enumerable keys of an object.
  *
- * @param {$ReadOnly<interface {[string]: mixed}>} obj The object for which the values are
+ * @param {object} obj The object for which the values are
  * to be returned.
  * @returns {Array<$Keys<O>>} An array of the enumerable keys of an object.
  */
 // NOTE(kevinb): This type was copied from TypeScript's library definitions.
-// eslint-disable-next-line @typescript-eslint/ban-types
-export function keys(obj: {}): string[] {
-    return Object.keys(obj);
+export function keys<O extends object>(obj: O): Array<keyof O> {
+    return Object.keys(obj) as Array<keyof O>;
 }