Prepare for 4.4.1

Bump version to 4.4

Add "SkipLastWhile" extension

Allow use of C# 13

Update package validation baseline version

Hide "CountBy" in .NET 9+

Update .NET SDK to 9, including dependencies

Fix x-plat shell use in CI build

Skip EC-linting of imported installation scripts

Align EC linting across CI builds

Add more OS + IDE files to ignore for version-tracking

Use collection expressions in tests

Update NUnit packages to their latest versions

Fix NUnit warnings NUnit1029 & NUnit1030

Add generic version of "Sequence" for any number type

Simplify internal implementation details of "EndsWith"

Merge remote-tracking branch 'upstream/master' into simpler-unfold

Drop conditional testing on Mono (#1081)

Drop CI build matrix & testing on macOS images

Update .NET SDK to 8.0.400, including dependencies

Update all tools to their latest versions

Bump version to 4.3.1

Bump version to 4.3

Enable Native AOT compatibility (trimming support)

Pin to .NET SDK 8.0.2xx

Remove testing against .NET 7 (near end-of-life)

Update NuGet package validation tool to latest version

Use static iterator methods to avoid closure

Update packages to their latest versions

Fix style errors: IDE0028, IDE0290, IDE0300 & IDE0301