[not-found] Add global-not-found convention (#78783)
### Feature
Introducing a new convention `global-not-found.js`, which let you build your own 404 page within app router. This convention is similar to `global-error.js` but for not found in general case, it's similar to `not-found.js` but including the layout.
App router has an implicit convention of building the 404 route, which is composing the top-level root `layout.js` and the root `not-found.js` to build up a 404 page. Since it's so implicit so that users don't have much control over it, which also lead to some confusing errors like `/_not-found` route is build failed due to various reason.
The `global-not-found.js` convention is aimed to help users to build 404 route in App Router easily and convenience. Currently it will be bound with a flag `experimental.globalNotFound` and later it will become the stable feature in next major release as it introduces a minor breaking change that we won't use root layout and root not-found to compose the 404 page. Instead, we'll either choose the builtin 404 page or users customized `global-not-found.js` to generate the 404 page when you enable the feature.
#### Advantages
Good new that if you were having struggles with creating 404 page for the multi root layouts app, such as the case below. Then `global-not-found.js` is your choose.
```
- (2024)/
/layout.js
/... # routes
- (2025)/
/layout.js
/... # routes
```
Closes NDX-1036
Related #59180
#### Follow up
The metadata and deployment will be followed up in new PRs
