core: cleanup sysgpu feature flag logic

* `@import("mach").core.gpu` has been renamed to `@import("mach").gpu`
* `pub const SYSGPUInterface` now has a default value (i.e. you do not need to write it in your main.zig, if you were.)
* You can now check `if (comptime mach.use_sysgpu)` for any conditional code you might have that should only run with sysgpu.

This (old):

```
pub const mach_core_options = core.ComptimeOptions{
    .use_wgpu = false,
    .use_sysgpu = true,
};
```

Has been replaced by this:

```
pub const use_sysgpu = true;
```

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

src/core/examples/sysgpu/instanced-cube/main.zig

@@ -24,10 +24,8 @@ bind_group: *gpu.BindGroup,
 
 pub const App = @This();
 
-pub const mach_core_options = core.ComptimeOptions{
-    .use_wgpu = false,
-    .use_sysgpu = true,
-};
+// Use experimental sysgpu graphics API
+pub const use_sysgpu = true;
 
 pub fn init(app: *App) !void {
     try core.init(.{});