This CL reorders the initialization scheme for shared and client
Isolates such that clients attach to the shared Isolate before
setting up the Heap. This is to support sharing the string table.

Bug: v8:12007
Change-Id: Icb0e40cc5ed84d516c8073a70d0f769f517044c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3039264
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77307}