mimalloc vendor snapshot

Source: https://github.com/microsoft/mimalloc
Commit: e2e6e70f8333e58ed0116591ca33a56227db9139
Release: 3.3.1

Notes:
- Copied include/, src/, LICENSE, readme.md, and Windows x86/x64
  mimalloc-redirect DLL/import libs from upstream.
- Removed upstream prebuilt binaries under bin/, except Windows
  x86/x64 mimalloc-redirect DLL/import libs (stored under redirect/)
  to enable CRT malloc/free redirection when MOZ_MIMALLOC is on.
- Added mozbuild integration via moz.build for static.c and exports.
- Set MI_DEFAULT_PURGE_DECOMMITS=0, MI_DEFAULT_PURGE_DELAY=500, and
  MI_DEFAULT_GENERIC_COLLECT=50000 in moz.build to preserve Basilisk's
  allocator tuning.
- Set MI_USE_ENVIRON=0 in moz.build so Unix/macOS builds use getenv only
  after preloading, avoiding loader-time crashes from raw environ traversal.
- On FreeBSD/DragonFly, wrap sysctl-related headers in default visibility
  to avoid hidden-visibility header issues.
- Guarded `mi_usable_size` to return 0 for non-mimalloc pointers to avoid early-init crashes.
- Darwin build uses MI_OSX_ZONE without MI_OSX_INTERPOSE to avoid duplicate
  interpose definitions.
- Windows exports mi_* symbols from mozglue to satisfy mozalloc inlines.
