Enable v8 snapshots

There are two layers here. The first is that, on startup, a v8 SnapshotCreator
is created, and a snapshot-specific isolate/context is setup with our browser
environment. This contains most of what was in Env.init and a good chunk of
what was in ExecutionWorld.createContext. From this, we create a v8.StartupData
which is used for the creation of all subsequent contexts. The snapshot sits
at the application level, above the Env - it's re-used for all envs/isolates, so
this gives a nice performance boost for both 1 connection opening multiple pages
or multiple connections opening 1 page.

The second layer is that the Snapshot data can be embedded into the binary, so
that it doesn't have to be created on startup, but rather created at build-time.
This improves the startup time (though, I'm not really sure how to measure that
accurately...).

The first layer is the big win (and just works as-is without any build / usage
changes).

with snapshot
total runs 1000
total duration (ms) 7527
avg run duration (ms) 7
min run duration (ms) 5
max run duration (ms) 41

without snapshot
total runs 1000
total duration (ms) 9350
avg run duration (ms) 9
min run duration (ms) 8
max run duration (ms) 42

To embed a snapshot into the binary, we first need to create the snapshot file:

zig build -Doptimize=ReleaseFast snapshot_creator -- src/snapshot.bin

And then build using the new snapshot_path argument:

zig build -Dsnapshot_path=../../snapshot.bin -Doptimize=ReleaseFast

The paths are weird, I know...since it's embedded, it needs to be inside the
project path, hence we put it in src/snapshot.bin. And since it's embedded
relative to the embedder (src/browser/js/Snapshot.zig) the path has to be
relative to that, hence ../../snapshot.bin. I'm open to suggestions on
improving this.

src/cdp/cdp.zig

@@ -22,7 +22,6 @@ const json = std.json;
 
 const log = @import("../log.zig");
 const js = @import("../browser/js/js.zig");
-const polyfill = @import("../browser/polyfill/polyfill.zig");
 
 const App = @import("../App.zig");
 const Browser = @import("../browser/Browser.zig");
@@ -700,10 +699,6 @@ const IsolatedWorld = struct {
     executor: js.ExecutionWorld,
     grant_universal_access: bool,
 
-    // Polyfill loader for the isolated world.
-    // We want to load polyfill in the world's context.
-    polyfill_loader: polyfill.Loader = .{},
-
     pub fn deinit(self: *IsolatedWorld) void {
         self.executor.deinit();
     }
@@ -729,7 +724,6 @@ const IsolatedWorld = struct {
         _ = try self.executor.createContext(
             page,
             false,
-            js.GlobalMissingCallback.init(&self.polyfill_loader),
         );
     }