This change defines a way that v8_debug_helper can describe object
fields which are packed structs, and uses it for the "descriptors" field
in DescriptorArray.

In more detail:
- debug-helper.h (the public interface for v8_debug_helper) adds a size
  and an optional list of struct properties to ObjectProperty.
- debug-helper-internal.h mirrors those changes to the internal class
  hierarchy which maintains proper unique_ptr ownership.
- In src/torque/class-debug-reader-generator.cc,
  - Some existing logic is moved into smaller functions.
  - New logic is added to generate the field list for structs. Example
    output is included in a comment above the function
    GenerateGetPropsChunkForField.

Bug: v8:9376
Change-Id: I531acac039ccb42050641448a4cbaec26186a7bc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1894362
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65079}