webapi: experimental (and useless, for now) ServiceWorker

This adds the shell for ServiceWorker, behind
`--experimental-features serviceworker`.

It's pretty useless as-is. We don't have the CacheStorage API (next) and don't
have the fetch interceptor (next next). But as-is, the change is quite big but
thankfully largely isolated.

src/Config.zig

@@ -245,8 +245,9 @@ pub const LoadResources = packed struct(u4) {
     stylesheet: bool = false,
 };
 
-const ExperimentalFeatures = packed struct(u1) {
+pub const ExperimentalFeatures = packed struct(u2) {
     cors: bool = false,
+    serviceworker: bool = false,
 };
 
 /// Common CLI args.