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>

.gitmodules

@@ -1,3 +1,3 @@
-[submodule "webgpu/libs/dawn"]
+[submodule "gpu/libs/dawn"]
-	path = webgpu/libs/dawn
+	path = gpu/libs/dawn
 	url = https://github.com/hexops/dawn

gpu/build.zig

@@ -7,7 +7,7 @@ pub fn build(b: *Builder) void {
     const mode = b.standardReleaseOptions();
     const target = b.standardTargetOptions(.{});
 
-    const lib = b.addStaticLibrary("webgpu", "src/main.zig");
+    const lib = b.addStaticLibrary("gpu", "src/main.zig");
     lib.setBuildMode(mode);
     lib.setTarget(target);
     lib.install();