Add a default_global_allocator feature so embedders can keep their own global allocator

oxc-node's Rust crate can be linked into another Node-API addon. Vite+ does this: it compiles oxc-node into its own native binding, next to rolldown_binding, so vpx script.ts can run oxc-node's hooks from the binary it already ships. That binary already has a global allocator, because rolldown_binding declares mimalloc. oxc-node declares one unconditionally (src/lib.rs#L35-L42), and a binary can only have one, so the build fails with rustc's error that the #[global_allocator] conflicts with the global allocator in another crate.

Proposal

Gate the allocator behind a feature, as rolldown_binding does with default_global_allocator:

#[cfg(all(
    not(target_arch = "x86"),
    not(target_arch = "arm"),
    not(target_family = "wasm"),
    not(all(target_os = "windows", target_arch = "aarch64")),
    not(feature = "default_global_allocator")
))]
#[global_allocator]
static ALLOC: mimalloc_safe::MiMalloc = mimalloc_safe::MiMalloc;

[features]
# Keep the system allocator, for hosts that declare their own.
default_global_allocator = []

Using the same name as Rolldown means an addon that links both enables the same feature on each. The published @oxc-node/core builds keep mimalloc, since the feature is off by default.

Today Vite+ removes the block in a patch.

docs(vpx): keep `.cts` ESM support and link the oxc-node follow-ups