We cannot have empty Zig structs mapping to JS instances

An empty struct will share the same address as its sibling (1) which will cause
an collision in the identity map.

(1) - This depends on Zig's non-guaranteed layout, so the collision might not
be with its sibling, but rather some other [seemingly random] field.

src/browser/crypto/crypto.zig

@@ -21,6 +21,8 @@ const uuidv4 = @import("../../id.zig").uuidv4;
 
 // https://w3c.github.io/webcrypto/#crypto-interface
 pub const Crypto = struct {
+    _not_empty: bool = true,
+
     pub fn _getRandomValues(_: *const Crypto, into: RandomValues) !RandomValues {
         const buf = into.asBuffer();
         if (buf.len > 65_536) {

src/browser/css/css.zig

@@ -29,6 +29,8 @@ pub const Interfaces = .{
 
 // https://developer.mozilla.org/en-US/docs/Web/API/CSS
 pub const Css = struct {
+    _not_empty: bool = true,
+
     pub fn _supports(_: *Css, _: []const u8, _: ?[]const u8) bool {
         // TODO: Actually respond with which CSS features we support.
         return true;