webgpu: rename to just "gpu"

It's nicer to refer to this in code as `gpu`. Additionally, `webgpu` as a name
gives the impression this is for web only which is absolutely not true but could
understandably be very confusing to newcomers.

Solve both problems by renaming to just `gpu`, and (next) updating the README to
indicate what it does and why, then explain it's WebGPU after as more of an
implementation detail.

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

gpu/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);
+}