This introduces a new (inspector-only) `v8::debug::ScriptSource`,
which represents the source for a given `v8::debug::Script` (in
case of JavaScript it's a `v8::internal::String` while in case of
WebAssembly it's a `Managed<v8::internal::wasm::NativeModule>`).
Every `v8_inspector::V8DebuggerScript` now holds on weakly to the
`v8::debug::Script` and strongly to its `ScriptSource`, making it
possible to access the source even after the `Script` dies.

This is preliminary work to allow for the removal of the special
GC feature that a `WeakCallbackType::kFinalizer` callback can
resurrect the object (this change is split into a separate follow
up CL https://crrev.com/c/3497324).

Bug: chromium:1295659, chromium:1302195
Doc: https://bit.ly/v8-inspector-script-caching
Change-Id: I503d0d9283e2da392023f06f79b8ff35953e7935
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3494242
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79363}