feat(@jest/expect): Expose type of actual to Matchers

Matchers isn't as typed as some users would like (see jestjs#13334, jestjs#13812).
For users who want to customize it by extending the `Matchers`
interface, it's useful to have access to the type of `actual` (the
argument of `expect`) so you can do, say,
```ts
interface Matchers<R, T> {
    toTypedEqual(expected: T): R
}
```
This commit exposes it. The first-party matchers still have the same
types as before.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Features
 
+- `[@jest/expect]` provide type of `actual` as a generic argument to `Matchers` to allow better-typed extensions (#TBD)
+
 ### Fixes
 
 ### Chore & Maintenance

packages/expect/src/types.ts

@@ -134,7 +134,7 @@ type PromiseMatchers = {
   resolves: Matchers<Promise<void>> & Inverse<Matchers<Promise<void>>>;
 };
 
-export interface Matchers<R extends void | Promise<void>> {
+export interface Matchers<R extends void | Promise<void>, T = unknown> {
   /**
    * Ensures the last call to a mock function was provided specific args.
    */

packages/jest-expect/src/types.ts

@@ -29,7 +29,7 @@ type Inverse<Matchers> = {
   not: Matchers;
 };
 
-type JestMatchers<R extends void | Promise<void>, T> = Matchers<R> &
+type JestMatchers<R extends void | Promise<void>, T> = Matchers<R, T> &
   SnapshotMatchers<R, T>;
 
 type PromiseMatchers<T = unknown> = {