Update & fix C# blocks under tutorials/scripting/

- `_Process` and `_PhysicsProcess` take their delta argument as a `double`
- Use string interpolation instead of concatenating strings
- Use `string` instead of `String` (`using System` was removed from everywhere in a previous PR)
- Use `System.Array.Empty<T>()` to init empty arrays
- Use `SignalName.*` instead of `nameof(*)` for signals
- Do not use `event` as an argument name (that's a keyword)
- Match node paths to screenshots when retrieving nodes from the tree
- Add a C# example for scene unique nodes
- Update the "Cross language scripting" page
- Add a note about using `is` against `null`

tutorials/scripting/c_sharp/c_sharp_differences.rst

@@ -466,7 +466,7 @@ GDScript                C#
 ``PackedByteArray``      ``byte[]``
 ``PackedFloat32Array``   ``float[]``
 ``PackedFloat64Array``   ``double[]``
-``PackedStringArray``    ``String[]``
+``PackedStringArray``    ``string[]``
 ``PackedColorArray``     ``Color[]``
 ``PackedVector2Array``   ``Vector2[]``
 ``PackedVector3Array``   ``Vector3[]``