Maintain user-level experimental-feature configuration separately for preview and stable versions

Related: #14862

Summary of the new feature/enhancement

Currently, selective enabling or disabling experimental features at the user level (-Scope CurrentUser, which is the default) takes effect for all side-by-side PowerShell (Core) installations, irrespective of whether they’re stable versions or preview versions.

This can unwittingly result in the following scenario:

• By selectively disabling an experimental feature in a preview version, you’re implicitly enabling experimental features for side-by-side stable versions, because the same powershell.config.json file is used by all side-by-side version, and because disabling is based on excluding the disabled feature from the list of explicitly enumerated enabled features in that file (based on the all-enabled starting point in preview versions).

The state of experimental features for preview versions should be managed separately from stable versions.

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An additional question is how to handle cross-preview-version configuration:

• Say you disable an experimental feature in a preview version (where all features are enabled by default).

• You later install a new preview version that comes with new experimental features.

• With a version-agnostic configuration, it isn’t just the previously explicitly disabled feature that ends up disabled, but all newly added ones as well.

Proposed implementation:

Introduce a new property in the user-level powershell.config.json file:

• named something like DisabledPreviewExperimentalFeatures, which is only read by preview versions, instead of the ExperimentalFeatures property.

• whose logic is the inverse of ExperimentalFeatures: all features except the ones listed should then be enabled.

Written as of PowerShell Core 7.2.0-preview.3

[daxian-dbw]

The current-user scope configuration file doesn’t come with PowerShell package. It’s created by user (explicitly, or via Enable-ExperimentalFeature command), so I don’t think this will affect MU.

[daxian-dbw]

@doctordns Consider there may be non-PowerShell-built-in modules in the module path that define experimental features, it’s not possible to list all experimental features until at PowerShell run time.