Make Named Tuples experimental feature again

[WojciechMazur]

During the core meeting 27.11.2024, we decided that Named Tuples should not be made stable yet.
This feature should remain experimental in 3.6, and should become available as a preview feature in 3.7.
We need to revert stablisation changes made to Named Tuples.
These changes need to be backported to 3.6.2 and 3.6.3 (already being cutoff and waiting for 3.6.2 release)

The preview feature is to be implemented (in 3.6.4) mode for features to are being stabilized and might become stable in next minor release. This would create a 3 stages for any new feature:

• experimental - (stage 1) requiring experimental compiler mode and feature flag, might change drastically or be removed at any moment
• preview - (stage 2) - viral, requires only preview flag mode, unlikely to change, can be used where binary compatibility is not required (eg. end user applications), not yet recommnded for libraries
• stable (stage 3) - fully stable, guaranteed to not change, fully binary compatible

Additionally, we can consider stage 0 for proof of concept features - a new feature implemented in the compiler behind a flag but requiring SIP approval

[OndrejSpanel]

27.10.2024

Suggested edit: 27.11.2024