Reduce allocations in .NET 9

[GerardSmit]

In .NET 9 it's now possible to get a dictionary value with a ReadOnlySpan<char>.

We're using a dictionary with lookup in the following places:

1. ZipArchiveFileSystem - Checking if the parent dictionary exists
2. MemoryFileSystem - When getting the node, we split the path and enumerate through it

In this PR I changed these lookups to a lookup with ReadOnlySpan<char>, so we're allocating less memory.

Project changes

• Added .NET 9 as target framework
• Changed the version to 9.0.100 in global.json
• Added PackageReference to System.Memory for .NET Standard and .NET Framework
• Changed lang version to 13, so we can use ref structs.

New API's

• UPathExtensions.GetDirectoryAsSpan(this UPath path) - Gets the dictionary as a span
• UPathExtensions.SpanSplit(this UPath path) - Returns an enumerator which returns the sections of the path as span

[xoofx]

Nice, thanks!