build(psalm): Configure unstable namespace

[provokateurin]

Summary

• Add separate psalm config for NCU
• Add NcuExperimentalChecker which is a copy of the OcpSinceChecker but checks for @experimental and not @since
• Bump psalm error level to 1 since we only write interfaces in this namespace and level 3, 2 and 1 provide some stricter checks for that (see https://psalm.dev/docs/running_psalm/error_levels/)
• No baseline for this namespace, we don't want to introduce any legacy code right from the start

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[nickvergessen]

missing check for since

[provokateurin]

For NCU we only want to check @experimental and not @since.
If the API is moved to OCP it is not compatible anyway due to the namespace change (even if everything else stays the same), so that's the point when the @since annotations need to be added and the @experimental annotations need to be removed.

[nickvergessen]

We need the since to be able to remember the 1 release until decision and 2 releases until removal?

https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/api.html

An API can only live in this unstable namespace for one major release.
…
Deprecated API from the NCU namespace are kept for 2 major releases.

[provokateurin]

I thought we'd add @experimental 31.0.0 to indicate the version it lives in the namespace.
@ArtificialOwl maybe you can shed some more light on how this should work, the documentation doesn't seem to be clear enough about this (please also update it if necessary).

[ArtificialOwl]

As you are saying, having @since while in unstable might be confusing, and we can follow the implementation version via @experimental