rename grid to a renderer, extract to subfolder

"Grid" was a really antiquated name when it had both the screen state
AND the renderering functionality tied together. This hasn't been true
for a LONG time and it is long overdue that this is renamed to its
proper name.

This also begins setting up a folder structure to anticipate future
renderers and rendering functionality. I'm not working on any alternate
renderers right now so the interface isn't expected to be good, just
laying out the files in this way.

src/renderer.zig

@@ -0,0 +1,14 @@
+//! Renderer implementation and utilities. The renderer is responsible for
+//! taking the internal screen state and turning into some output format,
+//! usually for a screen.
+//!
+//! The renderer is closely tied to the windowing system which usually
+//! has to prepare the window for the given renderer using system-specific
+//! APIs. The renderers in this package assume that the renderer is already
+//! setup (OpenGL has a context, Vulkan has a surface, etc.)
+
+pub const OpenGL = @import("renderer/OpenGL.zig");
+
+test {
+    @import("std").testing.refAllDecls(@This());
+}