This makes the installation sequence of WebAssembly.Tag slightly
shorter, slightly faster, slightly cleaner in corner-case semantics,
and slightly better documented.

To allow testing this code, Isolate::InstallConditionalFeatures is
exposed as d8.test.installConditionalFeatures().

Fixed: chromium:1314616
Change-Id: I44285e398b8797e0e7d2d8c782cecec3ba68a503
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3582382
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79956}