Make js.Bridge aware of string.String for input parameters

Avoids having to allocate small strings when going from v8 -> Zig. Also
added a discriminatory type, string.Global which uses the arena, rather than
the call_arena, if an allocation _is_ necessary. (This is similar to a feature
we had before, but was lost in zigdom). Strings from v8 that need to be
persisted, can be allocated directly v8 -> arena, rather than v8 -> call_arena
-> arena.

I think there are a lot of places where we should use string.String - where
strings are expected to be short (e.g. attribute names). But started with just
document.querySelector and querySelectorAll.

src/string.zig

@@ -188,6 +188,13 @@ pub const String = packed struct {
     }
 };
 
+// Discriminatory type that signals the bridge to use arena instead of call_arena
+// Use this for strings that need to persist beyond the current call
+// The caller can unwrap and store just the underlying .str field
+pub const Global = struct {
+    str: String,
+};
+
 fn asUint(comptime string: anytype) std.meta.Int(
     .unsigned,
     @bitSizeOf(@TypeOf(string.*)) - 8, // (- 8) to exclude sentinel 0