AK: Adopt mimalloc v2 as main allocator

Use mimalloc for Ladybird-owned allocations without overriding malloc().
Route kmalloc(), kcalloc(), krealloc(), and kfree() through mimalloc,
and put the embedded Rust crates on the same allocator via a shared
shim in AK/kmalloc.cpp.

This also lets us drop kfree_sized(), since it no longer used its size
argument. StringData, Utf16StringData, JS object storage, Rust error
strings, and the CoreAudio playback helpers can all free their AK-backed
storage with plain kfree().

Sanitizer builds still use the system allocator. LeakSanitizer does not
reliably trace references stored in mimalloc-managed AK containers, so
static caches and other long-lived roots can look leaked. Pass the old
size into the Rust realloc shim so aligned fallback reallocations can
move posix_memalign-backed blocks safely.

Static builds still need a little linker help. macOS app binaries need
the Rust allocator entry points forced in from liblagom-ak.a, while
static ELF links can pull in identical allocator shim definitions from
multiple Rust staticlibs. Keep the Apple -u flags and allow those
duplicate shim symbols for LibJS and LibRegex links on Linux and BSD.

vcpkg.json

@@ -113,6 +113,7 @@
       "name": "mman",
       "platform": "windows"
     },
+    "mimalloc",
     "openssl",
     {
       "name": "pthread",
@@ -309,6 +310,10 @@
       "name": "libxml2",
       "version": "2.13.8#1"
     },
+    {
+      "name": "mimalloc",
+      "version": "2.2.7"
+    },
     {
       "name": "mman",
       "version": "git-f5ff813#5"