Enable v8 snapshots

[karlseguin]

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.