This is the last of the big changes to the js code

This Pr largely tightens up a lot of the code. 'v8' is no longer imported
outside of js. A number of helper functions have been moved to the js.Context.
For example, js.Function.getName used to call:

```zig
return js.valueToString(allocator, name, self.context.isolate, self.context.v8_context);
```

It now calls:

```zig
return self.context.valueToString(name, .{ .allocator = allocator });
```

Page.main_context has been renamed to `Page.js`. This, in combination with new
promise helpers, turns:

```zig
const resolver = page.main_context.createPromiseResolver();
try resolver.resolve({});
return resolver.promise();
```

into:

```zig
return page.js.resolvePromise({});
```

src/cdp/cdp.zig

@@ -690,9 +690,7 @@ const IsolatedWorld = struct {
             return;
         }
         _ = try self.executor.createContext(
-            &page.window,
             page,
-            null,
             false,
             js.GlobalMissingCallback.init(&self.polyfill_loader),
         );

src/cdp/domains/dom.zig

@@ -274,7 +274,7 @@ fn resolveNode(cmd: anytype) !void {
     const bc = cmd.browser_context orelse return error.BrowserContextNotLoaded;
     const page = bc.session.currentPage() orelse return error.PageNotLoaded;
 
-    var js_context = page.main_context;
+    var js_context = page.js;
     if (params.executionContextId) |context_id| {
         if (js_context.v8_context.debugContextId() != context_id) {
             for (bc.isolated_worlds.items) |*isolated_world| {

src/cdp/domains/page.zig

@@ -251,7 +251,7 @@ pub fn pageNavigate(arena: Allocator, bc: anytype, event: *const Notification.Pa
         const page = bc.session.currentPage() orelse return error.PageNotLoaded;
         const aux_data = try std.fmt.allocPrint(arena, "{{\"isDefault\":true,\"type\":\"default\",\"frameId\":\"{s}\"}}", .{target_id});
         bc.inspector.contextCreated(
-            page.main_context,
+            page.js,
             "",
             try page.origin(arena),
             aux_data,

src/cdp/domains/target.zig

@@ -147,7 +147,7 @@ fn createTarget(cmd: anytype) !void {
     {
         const aux_data = try std.fmt.allocPrint(cmd.arena, "{{\"isDefault\":true,\"type\":\"default\",\"frameId\":\"{s}\"}}", .{target_id});
         bc.inspector.contextCreated(
-            page.main_context,
+            page.js,
             "",
             try page.origin(cmd.arena),
             aux_data,