- Convert gcmole to python3-ish code
  - use local Path implementation for future full migration
- Use optparse and explicit arguments for gcmole
  - Add explicit directories flags
  - Use backwards compatible env vars as fallbacks
- Add gn target v8_gcmole_files to avoid issues with missing or
  incompatible generated files

Drive-by-fixes for running gcmole without ignored files:
- Disable gcmole in Isolate::UnwindAndFindHandle
- Partially disable gcmole in V8HeapExplorer::AddEntry

Bug: v8:10009
Change-Id: I5b4d1c6554db300190226361b6c518419109ff3d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497320Reviewed-by: Liviu Rau <liviurau@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79348}