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Has been replaced by this: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/platform/wasm/entrypoint.zig

@@ -8,8 +8,10 @@ pub usingnamespace @import("app");
 const App = @import("app").App;
 
 const std = @import("std");
-const core = @import("mach").core;
-const gpu = core.gpu;
+
+const mach = @import("mach");
+const core = mach.core;
+const gpu = mach.gpu;
 
 pub const GPUInterface = gpu.StubInterface;