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/App.zig

@@ -22,6 +22,7 @@ const Allocator = std.mem.Allocator;
 
 const log = @import("log.zig");
 const Http = @import("http/Http.zig");
+const Snapshot = @import("browser/js/Snapshot.zig");
 const Platform = @import("browser/js/Platform.zig");
 
 const Notification = @import("Notification.zig");
@@ -34,6 +35,7 @@ const App = @This();
 http: Http,
 config: Config,
 platform: Platform,
+snapshot: Snapshot,
 telemetry: Telemetry,
 allocator: Allocator,
 app_dir_path: ?[]const u8,
@@ -83,6 +85,9 @@ pub fn init(allocator: Allocator, config: Config) !*App {
     app.platform = try Platform.init();
     errdefer app.platform.deinit();
 
+    app.snapshot = try Snapshot.load(allocator);
+    errdefer app.snapshot.deinit(allocator);
+
     app.app_dir_path = getAndMakeAppDir(allocator);
 
     app.telemetry = try Telemetry.init(app, config.run_mode);
@@ -101,6 +106,7 @@ pub fn deinit(self: *App) void {
     self.telemetry.deinit();
     self.notification.deinit();
     self.http.deinit();
+    self.snapshot.deinit(allocator);
     self.platform.deinit();
 
     allocator.destroy(self);