rename mach/gpu to mach/gpu-dawn

See https://github.com/hexops/mach/issues/133#issuecomment-999940767

Helps #109
Helps #133

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu-dawn/src/main.zig

@@ -0,0 +1,10 @@
+const std = @import("std");
+const testing = std.testing;
+
+export fn add(a: i32, b: i32) i32 {
+    return a + b;
+}
+
+test "basic add functionality" {
+    try testing.expect(add(3, 7) == 10);
+}