[wasm] Factorize wasm-js constructor setup

R=ahaas@chromium.org

Change-Id: I860cd7aebdf46f323c7f6ad0cd563246a061c4d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226338
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77525}