Add PROPERTY_USAGE_NEVER_DUPLICATE flag and use for script

[aaronfranke]

This PR fixes #33079 and supersedes #41923.

This PR is split into two commits:

• PropertyUsage: Rename "DO_NOT_SHARE_ON_DUPLICATE" to "ALWAYS_DUPLICATE"
  • While working on this PR, I found the old name to be confusing. The action that can be done or not is duplication, the term "share" is mostly foreign terminology to this process. Sharing means "do not duplicate", so "do not share" means "do not not duplicate". I think it makes a lot more sense with the name "ALWAYS_DUPLICATE". Technically it could be "always duplicate on duplicate" but that seems unnecessary and redundant.
  • This part breaks compatibility. I can later make a 3.x version of this PR without this rename.
• Add PROPERTY_USAGE_NEVER_DUPLICATE flag and use for script
  • This is the part that fixes Calling duplicate(true) on custom Resource returns empty Resource. #33079 and supersedes Add PROPERTY_USAGE_ALWAYS_SHARE_ON_DUPLICATE flag #41923.
  • In addition to what's in 41923, this commit also exposes this value and improves the documentation.
  • Tested on the below minimal reproduction project.

Minimal reproduction project in GDScript: test_33079.zip

[raulsntos]

Changes to the PropertyUsageFlags enum need to be synchronized with the enum in C# source generators:

godot/modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/GodotEnums.cs

Line 93 in 91713ce

internal enum PropertyUsageFlags

[akien-mga]

Approved by production team.

We thought it would be worth furthering the compat breakage a bit by moving NEVER_DUPLICATE next to ALWAYS_DUPLICATE in the enum, so they're adjacent in the docs.

[kleonc]

This likely fixes #68666 (only for Resources stored/duplicated after this PR; if something was already saved with script then it would still fail loading).

[akien-mga]

Thanks!