Improvement: FindFileOrThrow for dotnet tools caching result between project sessions

Hi, handzlikchris. Thank you for your excellent work.

I noticed that every time project opens, FSR always search deeply into Unity installation path for dotnet tools, which spends seconds to complete. Because of storing in `SessionState`, first-time project launch can't benefit from it.

In my opinion, utilize `EditorPrefs` with `unityVersion` could be better.

Here is my snippet for this:

```csharp
private static string FindFileOrThrow(string fileName)
{
var cacheKey = $"FSR:FilePath_{fileName}:{Application.unityVersion}";
var result = EditorPrefs.GetString(cacheKey, null);

if (result != null && !File.Exists(result))
{
EditorPrefs.DeleteKey(cacheKey);
result = null;
}

if (result == null)
{
result = Directory
.GetFiles(ApplicationContentsPath, fileName, SearchOption.AllDirectories)
.FirstOrDefault();

if (result == null)
{
throw new Exception($"Unable to find '{fileName}', make sure Editor version supports it. You can also add preprocessor directive 'FastScriptReload_CompileViaMCS' which will use Mono compiler instead");
}
}

EditorPrefs.SetString(cacheKey, result);
return result;
}

```