Support all ImmutableArray functions as in Array module

[xperiandri]

Implemented support for all the functions as in similar FSharp.Core modules

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

[Copilot]

Pull Request Overview

This pull request adds comprehensive support for ImmutableArray functions modeled after the Array module, including a large suite of new tests and internal helper updates. Key changes include:

• New test files covering nearly every aspect of FlatList operations (creation, manipulation, iteration, folding, etc.) and additional builder functionality.
• Removal of the old ImmutableCollectionUtil.fs file with its functionality migrated into Helper.fs.
• Update of the test project file to target .NET 9.0.

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/FSharp.Collections.Immutable.Tests/Helpers.fs	Introduces helper functions (fstv, sndv) for testing.
tests/FSharp.Collections.Immutable.Tests/FlatList/UtilityTests.fs	Adds tests for FlatList functions; note a typo in a test name.
tests/FSharp.Collections.Immutable.Tests/FlatList/*.fs	New tests covering flat list creation, manipulation, sorting, etc.
tests/FSharp.Collections.Immutable.Tests/FSharp.Collections.Immutable.Tests.fsproj	Changes target framework from net8.0 to net9.0.
src/FSharp.Collections.Immutable/ImmutableCollectionUtil.fs	Removed internal file, functionality merged into Helper.fs.
src/FSharp.Collections.Immutable/Helper.fs	Now contains internal helper modules and utility functions.