Enable pixel snapping

In @theshaggydev's "Configuring your Godot project for pixel art"
[video][1] and [article][2], pixel snapping is enabled. The exact
setting used there isn't available in Godot 4, but I believe these
settings - or at least the vertices one - are at least roughly
equivalent. I may revert these settings if I notice weird behaviour
when building the game

[1]: https://www.youtube.com/watch?v=Nsj5SGEUZhU&t=155s
[2]: https://shaggydev.com/2021/09/21/project-setup-for-pixel-art/#pixel-snap

project.godot

@@ -27,3 +27,5 @@ window/stretch/mode="viewport"
 textures/canvas_textures/default_texture_filter=0
 renderer/rendering_method="gl_compatibility"
 renderer/rendering_method.mobile="gl_compatibility"
+2d/snap/snap_2d_transforms_to_pixel=true
+2d/snap/snap_2d_vertices_to_pixel=true