mimalloc vendor snapshot

Source: https://github.com/microsoft/mimalloc
Commit: b69f9cb39f22b95971a4b9196a9c265a7b474718
Release: 2.2.7

Notes:
- Copied include/, src/, LICENSE, and readme.md 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.
- Adjusted Unix getenv handling to avoid raw environ traversal during process init,
  using getenv only after preloading to prevent loader-time crashes.
- On GNU platforms, prefer weak `_environ` over `environ` to handle glibc DSO visibility.
- 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.