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Has been replaced by this: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/main.zig

@@ -4,7 +4,7 @@ const builtin = @import("builtin");
 // Core
 pub const core = if (build_options.want_core) @import("core/main.zig") else struct {};
 pub const Timer = if (build_options.want_core) core.Timer else struct {};
-pub const gpu = if (build_options.want_core) core.gpu else struct {};
+pub const wgpu = if (build_options.want_core) @import("gpu/main.zig") else struct {};
 pub const sysjs = if (build_options.want_core) @import("mach-sysjs") else struct {};
 pub const Core = if (build_options.want_core) @import("Core.zig") else struct {};
 
@@ -32,6 +32,14 @@ pub const Mod = ModSet(modules).Mod;
 pub const EntityID = @import("module/main.zig").EntityID; // TODO: rename to just Entity?
 pub const Archetype = @import("module/main.zig").Archetype;
 
+/// To use experimental sysgpu graphics API, you can write this in your main.zig:
+///
+/// ```
+/// pub const use_sysgpu = true;
+/// ```
+pub const use_sysgpu = if (@hasDecl(@import("root"), "use_sysgpu")) @import("root").use_sysgpu else false;
+pub const gpu = if (use_sysgpu) sysgpu.sysgpu else wgpu;
+
 test {
     const std = @import("std");
     // TODO: refactor code so we can use this here: