Bug: Removed Unity samples break UPM

[Mekyi]

ZString 2.6.0

Previously removed contents from Samples~/RequiredManagedDLLs/Plugins (commit 61447cf) are causing Unity Package Manager to hang when installing the package via git url due to following configuration in src/ZString.Unity/Assets/Scripts/ZString/package.json.

"samples": [
	{
		"displayName": "Required Managed DLLs",
		"description": "Import Required Managed DLLs",
		"path": "Samples~/RequiredManagedDLLs"
	}
]

This causes installation to get visually stuck and produces the following error in the console when Unity project is opened:

DirectoryNotFoundException: The path /Library/PackageCache/com.cysharp.zstring@ad8385676113/Samples~/RequiredManagedDLLs does not exist.```

[Mekyi]

Opened a PR with possible fix.