Support for `__builtins__.pyi`?

Feature

In #12860 @erictraut said:

We decided to support this in pyright through the use of a specially-named builtins.pyi stub file. If a stub with this name is found within the root directory of a project, pyright treats any symbols declared within that stub as though they are appended to the bottom of builtins.pyi.

and @AlexWaygood closed the issue saying:

if anybody wants to request a mypy equivalent of the more limited feature Eric's introduced over at pyright, they can open another issue to do so.

I would like to request a mypy version of this feature, so that custom Python distributions that provide extra builtins can ship type definitions that are understood by mypy. In specific, I am interested in this because PyScript is discussing adding a builtin called display which is like a web aware print and they want to make it a builtin.
https://github.com/pyscript/pyscript/discussions/1687

It's worth pointing out that typescript also has a version of this feature with declare global. Though the JavaScript ecosystem also has a lot of tools that pollute the "builtins"-level scope so they have a much stronger need for this sort of thing.

Read the referenced pyright docs/builtins.md and compare its builtins.pyi behavior with mypy's existing handling of builtins.pyi. Trace how a project root is discovered and how builtins definitions are loaded; done means a custom root-level stub contributes symbols to type checking, with tests covering the behavior.