Move caching logic out of the {WasmEngine} and in its own
{NativeModuleCache} class, with its own mutex.

R=clemensb@chromium.org

Bug: v8:6847
Change-Id: I73067fd9f0556e57c28782088dcb772a14265154
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2004613Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65895}