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.

build.zig

@@ -29,10 +29,12 @@ pub fn build(b: *Build) !void {
 
     const git_commit = b.option([]const u8, "git_commit", "Current git commit");
     const prebuilt_v8_path = b.option([]const u8, "prebuilt_v8_path", "Path to prebuilt libc_v8.a");
+    const snapshot_path = b.option([]const u8, "snapshot_path", "Path to v8 snapshot");
 
     var opts = b.addOptions();
     opts.addOption([]const u8, "version", manifest.version);
     opts.addOption([]const u8, "git_commit", git_commit orelse "dev");
+    opts.addOption(?[]const u8, "snapshot_path", snapshot_path);
 
     // Build step to install html5ever dependency.
     const html5ever_argv = blk: {
@@ -112,6 +114,30 @@ pub fn build(b: *Build) !void {
         run_step.dependOn(&run_cmd.step);
     }
 
+    {
+        // snapshot creator
+        const exe = b.addExecutable(.{
+            .name = "lightpanda-snapshot-creator",
+            .use_llvm = true,
+            .root_module = b.createModule(.{
+                .root_source_file = b.path("src/main_snapshot_creator.zig"),
+                .target = target,
+                .optimize = optimize,
+                .imports = &.{
+                    .{ .name = "lightpanda", .module = lightpanda_module },
+                },
+            }),
+        });
+        b.installArtifact(exe);
+
+        const run_cmd = b.addRunArtifact(exe);
+        if (b.args) |args| {
+            run_cmd.addArgs(args);
+        }
+        const run_step = b.step("snapshot_creator", "Generate a v8 snapshot");
+        run_step.dependOn(&run_cmd.step);
+    }
+
     {
         // test
         const tests = b.addTest(.{