Instead of having e.g. `string.new_wtf8` that takes an immediate
specifying the particular UTF-8 flavor to parse, make one instruction
per flavor.

See https://github.com/WebAssembly/stringref/pull/46.

Bug: v8:12868
Change-Id: I2e9f2735c557b2352b6e75314037e473710d87a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3892695Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andy Wingo <wingo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#83170}