Make it easier to augment parts of HotAvalonia

 - `AvaloniaHotReloadAttribute` and `AvaloniaHotReloadExtensions` are partial now
 - It's now trivial to define a custom project path resolver: `static string? AvaloniaHotReloadExtensions.ResolveProjectPath(Assembly)`
 - It's now possible to use a custom file system provider:
    1) Define `HOTAVALONIA_USE_CUSTOM_FILE_SYSTEM`
    2) Implement `static IFileSystem AvaloniaHotReloadExtensions.GetFileSystem()`

src/HotAvalonia.Extensions/AvaloniaHotReloadExtensions.cs

@@ -71,15 +71,15 @@ namespace HotAvalonia
     [ExcludeFromCodeCoverage]
     [Conditional("HOTAVALONIA_ENABLE")]
     [AttributeUsage(AttributeTargets.Method)]
-    internal sealed class AvaloniaHotReloadAttribute : Attribute
+    internal sealed partial class AvaloniaHotReloadAttribute : Attribute
     {
     }
 
     /// <summary>
     /// Provides extension methods for enabling and disabling hot reload functionality for Avalonia applications.
     /// </summary>
     [ExcludeFromCodeCoverage]
-    internal static class AvaloniaHotReloadExtensions
+    internal static partial class AvaloniaHotReloadExtensions
     {
 #if HOTAVALONIA_ENABLE && !HOTAVALONIA_DISABLE
         /// <summary>
@@ -139,6 +139,7 @@ private static IHotReloadContext CreateHotReloadContext(AvaloniaProjectLocator p
 #endif
         }
 
+#if !HOTAVALONIA_USE_CUSTOM_FILE_SYSTEM
         /// <summary>
         /// Gets the current file system instance.
         /// </summary>
@@ -152,6 +153,7 @@ private static IHotReloadContext CreateHotReloadContext(AvaloniaProjectLocator p
             return global::HotAvalonia.IO.FileSystem.Current;
 #endif
         }
+#endif
 #endif
 
         /// <summary>

src/HotAvalonia.Extensions/AvaloniaHotReloadExtensions.fs

@@ -128,6 +128,7 @@ type internal AvaloniaHotReloadExtensions =
         AvaloniaHotReloadContext.Create(projectLocator)
 #endif
 
+#if !HOTAVALONIA_USE_CUSTOM_FILE_SYSTEM
     /// <summary>
     /// Gets the current file system instance.
     /// </summary>
@@ -139,6 +140,7 @@ type internal AvaloniaHotReloadExtensions =
 #else
         HotAvalonia.IO.FileSystem.Current
 #endif
+#endif
 #endif
 
     /// <summary>

src/HotAvalonia.Extensions/AvaloniaHotReloadExtensions.vb

@@ -68,15 +68,15 @@ Namespace Global.HotAvalonia
     <ExcludeFromCodeCoverage>
     <Conditional("HOTAVALONIA_ENABLE")>
     <AttributeUsage(AttributeTargets.Method)>
-    Friend NotInheritable Class AvaloniaHotReloadAttribute
+    Friend NotInheritable Partial Class AvaloniaHotReloadAttribute
         Inherits Attribute
     End Class
 
     ''' <summary>
     ''' Provides extension methods for enabling and disabling hot reload functionality for Avalonia applications.
     ''' </summary>
     <ExcludeFromCodeCoverage>
-    Friend Module AvaloniaHotReloadExtensions
+    Friend Partial Module AvaloniaHotReloadExtensions
 #If HOTAVALONIA_ENABLE AndAlso Not HOTAVALONIA_DISABLE Then
         ''' <summary>
         ''' Creates a factory method for generating an <see cref="IHotReloadContext"/>
@@ -133,6 +133,7 @@ Namespace Global.HotAvalonia
 #End If
         End Function
 
+#If Not HOTAVALONIA_USE_CUSTOM_FILE_SYSTEM Then
         ''' <summary>
         ''' Gets the current file system instance.
         ''' </summary>
@@ -145,6 +146,7 @@ Namespace Global.HotAvalonia
             Return HotAvalonia.IO.FileSystem.Current
 #End If
         End Function
+#End If
 #End If
 
         ''' <summary>