Improve consistency of GetCacheDirectory API

[oleg-shilo]

Triggered by #441

Let me add a caching related question/issue. Feel free to extract into a separate issue if you think it doesn't belong here.

Runtime.cs restricts "does not support the cache directory for class library projects", resulting in a host application not being able to retrieve CacheDir. While it can replicate CacheDir = GetScriptTempDir().PathJoin("cache"), this obviously is something different than GetCacheDir(). I am not really getting this piece of code.

Or stated differently, could Runtime provide...

public static string GetCacheDir() => CacheDir;

...on #if class_lib?

Or is CSExecutor.GetCacheDirectory() something that could be made public?

Note the differences among naming of SetScriptCacheDir() vs. GetCacheDirectory(). Should probably be better aligned.

Also, note that with my host app CS-Script creates two differently named directories in "\Temp":

"C:\Users<USER>\AppData\Local\Temp\CSSCRIPT\cache" which contains build info of the scripts.
"C:\Users<USER>\AppData\Local\Temp\csscript.core" which remains empty.

Background: My host app outputs verbose information on compiling and loading a script, just like CS-Script -verbose does. But currently there is no method to retrieve the cache dir.

Originally posted by @maettu-this in #441

[oleg-shilo]

is CSExecutor.GetCacheDirectory(), something that could be made public?

Not exactly. CSExecutor is internal and it serves the internal implementation of teh CLI script engine.

I have refactored the codebase to address your concerns, which are quite valid. And this refactoring now makes API naming more consistent.
Though keep in mind that the convention *ScriptCache* vs *Cache* is designed to differentiate between the script file specific cache folder and the generic global cache folder of the CS-Script. It is reflected in teh API documentation.

Below is teh summary of the Caching and Temp dir management.

CSScriptLib:
  ┌─────────────┐
  │  CSScript   │ (Public API + Infrastructure)
  └─────────────┘

cscs:
  ┌──────────────┐      ┌─────────────────────────┐
  │  CSExecutor  │ ───► │   Runtime               │
  │              │      │     - Environment       │
  │              │      │     - Cache management  │
  │              │      │     - Temp directories  │
  └──────────────┘      └─────────────────────────┘

Responsibility Matrix

Responsibility	CSScript (CSScriptLib)	Runtime (cscs)
Script Evaluation	✅ Primary	❌ No
Compiler Abstraction	✅ Yes (Roslyn/CodeDom)	❌ No
OS Detection	❌ No	✅ Yes
.NET Version Detection	❌ No	✅ Yes
Temp Directories	✅ Yes	✅ Yes
Cache Management	✅ Yes	✅ Yes
Public API	✅ Yes	❌ Internal only

And this is the summary of the use of the temp folders:

C:\Users\<USER>\AppData\Local\Temp\
├── csscript\          # Legacy .NET Framework (deprecated)
├── csscript.core\     # Modern .NET CLI (cscs.exe, csws.exe)
└── csscript.lib\      # CSScriptLib (in-process script evaluation)

And this is the extended MD doc of teh summary above: https://github.com/oleg-shilo/cs-script/blob/master/src/docs/Temporary-Files-and-Cache.md
It's AI genearetd and I will merge it with Wiki after I review it.