gpu: add foundation for interface

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

gpu/src/Device.zig

@@ -0,0 +1,12 @@
+//! A standard interface to a WebGPU implementation.
+//!
+//! Like std.mem.Allocator, but representing a WebGPU implementation.
+
+// The type erased pointer to the Device implementation
+ptr: *anyopaque,
+vtable: *const VTable,
+
+pub const VTable = struct {
+    // TODO(gpu): make these *const fn once stage2 is released.
+    free: fn (ptr: *anyopaque, buf: []u8, buf_align: u29, ret_addr: usize) void,
+};