Update page.js based on the current context.

page.js currently always references the page context. But through the inspector
JavaScript can be executed in different contexts. When we go from V8->Zig we
correctly capture the current context within the caller's Local. And, because of
this, mapping or anything else that happens against local.ctx, happens in the
right context.

EXCEPT...our code still accesses page.js. So you can have a v8->zig call
happening in Context-2, and our Zig call then tries to do something on Context-1
via page.js.

I'm introducing a change that updates page.js based on the current Caller and
restores it at the end of the Caller. This change is super small, but
potentially has major impact. It's hard to imagine that we haven't run into
problems with this before, and it's hard to imagine what problems this change
might introduce. Certainly, if anyone copies page.js, they'll be in for a rude
surprise, but i don't think we do that anywhere.

src/browser/js/Caller.zig

@@ -35,6 +35,7 @@ const IS_DEBUG = @import("builtin").mode == .Debug;
 const Caller = @This();
 local: js.Local,
 prev_local: ?*const js.Local,
+prev_context: *Context,
 
 // Takes the raw v8 isolate and extracts the context from it.
 pub fn init(self: *Caller, v8_isolate: *v8.Isolate) void {
@@ -53,7 +54,9 @@ pub fn init(self: *Caller, v8_isolate: *v8.Isolate) void {
             .isolate = .{ .handle = v8_isolate },
         },
         .prev_local = ctx.local,
+        .prev_context = ctx.page.js,
     };
+    ctx.page.js = ctx;
     ctx.local = &self.local;
 }
 
@@ -79,6 +82,7 @@ pub fn deinit(self: *Caller) void {
 
     ctx.call_depth = call_depth;
     ctx.local = self.prev_local;
+    ctx.page.js = self.prev_context;
 }
 
 pub const CallOpts = struct {