This adds a new --experimental-value-unavailable flag, which is disabled
for now. When enabled the debugger reports values that are optimized out
by TurboFan and values of certain variables in Temporal Dead Zones (TDZ)
as unavailable. Internally we use a special `value_unavailable` accessor
info to represent these values, and on the debugger boundary we report
these properties with `value`, `get`, or `set`.

Doc: https://goo.gle/devtools-value-unavailable
Bug: chromium:1328681
Demo: devtools-dbg-stories.netlify.app/crbug-1328681-value-unavailable
Change-Id: Idb09a4a148335a950deae60f7c07caecc48826ba
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3627510
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81509}