movement: generalize; use vectors (#16)

Generalized movement, so that all entities move the same.

Reviewed-on: #16
Reviewed-by: raf <raf@notashelf.dev>
Co-authored-by: Squirrel Modeller <squirrelmodeller@protonmail.com>
Co-committed-by: Squirrel Modeller <squirrelmodeller@protonmail.com>

build.zig

@@ -25,6 +25,7 @@ pub fn build(b: *std.Build) void {
         "src/main.c",
         "src/map.c",
         "src/player.c",
+        "src/movement.c",
         "src/render.c",
         "src/rng.c",
         "src/settings.c",