Fix C# examples in documentation

- Fix documentation after C# renames.
- Add missing `partial` in C# class declarations.
- Change `delta` parameter type to `double` in C#.
- Ensure parameters match base declaration.
- Use `$` string interpolation in C#.
- Fix invalid or outdated C# code.
- Changed some examples to follow our style guide more closely.

doc/classes/Expression.xml

@@ -24,7 +24,7 @@
 		        $LineEdit.text = str(result)
 		[/gdscript]
 		[csharp]
-		public Expression expression = new Expression();
+		private Expression _expression = new Expression();
 
 		public override void _Ready()
 		{
@@ -33,14 +33,14 @@
 
 		private void OnTextEntered(string command)
 		{
-		    Error error = expression.Parse(command);
+		    Error error = _expression.Parse(command);
 		    if (error != Error.Ok)
 		    {
-		        GD.Print(expression.GetErrorText());
+		        GD.Print(_expression.GetErrorText());
 		        return;
 		    }
-		    object result = expression.Execute();
-		    if (!expression.HasExecuteFailed())
+		    Variant result = _expression.Execute();
+		    if (!_expression.HasExecuteFailed())
 		    {
 		        GetNode<LineEdit>("LineEdit").Text = result.ToString();
 		    }