Add custom allocator support to ty with memory stats reporting
- Copy Ruff's platform-specific allocator configuration to ty:
- Windows: uses mimalloc
- Unix-like (x86_64, aarch64, powerpc64, riscv64): uses jemalloc by default
- Other platforms: uses system allocator
- Add `mimalloc` feature flag to prefer mimalloc over jemalloc on
platforms that support both allocators
- Add allocator memory usage statistics:
- Set TY_ALLOCATOR_STATS=1 to print memory stats on exit
- jemalloc: shows allocated, active, resident, mapped, retained,
metadata bytes and fragmentation percentage
- mimalloc: provides guidance for using MIMALLOC_SHOW_STATS=1
- Add tikv-jemalloc-ctl workspace dependency with stats feature
