Distribute stdlib types as types-stdlib

This has been briefly discussed before, but I think it would be a good idea if we started distributing the stdlib types in a separate package types-stdlib, similar to third-party packages:

• Type checkers could optionally depend on (a specific version of) that package, instead of shipping their own copy, reducing coupling and arguably improving maintainability.
• Third-party stubs could require a minimum version of the types-stdlib package, for example if they are relying on _typeshed features or for cases like #5762. Type checkers that opt to distribute a copy of the stdlib, could warn if incompatible stubs are used.

Technical considerations:

• The versioning scheme would be similar to third-party stubs, e.g. types-stdlib 3.10.X, where X is incremented automatically. We should probably increment the version number as soon as the first beta is released and we are reasonably sure we have added all new/changed API.
• The stubs would be installed into a special directory, for example stdlib, or even a non-importable name like types-stdlib (so we'd have stubs stdlib/sys.pyi etc.) so that they are only used by type checkers that support this package and don't interfere.

Start by reviewing the issue discussion and prior linked context, since no files, tests, or entry points are named. Define the package layout, versioning scheme, installation behavior, and compatibility expectations before implementation; done requires agreement on the design and a complete distribution path for the stdlib stubs.