feat(jest-config): add defineConfig and mergeConfig functions (#15844)

Author: ahnpnl

CHANGELOG.md

@@ -1,6 +1,10 @@
 ## main
 
-## Fixes
+### Features
+
+- `[jest-config]` Add `defineConfig` and `mergeConfig` helpers for type-safe Jest config ([#15844](https://github.com/jestjs/jest/pull/15844))
+
+### Fixes
 
 - `[jest-runtime]` Fix issue where user cannot utilize dynamic import despite specifying `--experimental-vm-modules` Node option ([#15842](https://github.com/jestjs/jest/pull/15842))