Use mimalloc as default allocator (#7140)

From https://github.com/microsoft/mimalloc:
> In our benchmarks (see
[below](https://github.com/microsoft/mimalloc#performance)), mimalloc
outperforms other leading allocators (jemalloc, tcmalloc, Hoard, etc),
and often uses less memory. A nice property is that it does consistently
well over a wide range of benchmarks. There is also good huge OS page
support for larger server programs.


Release Notes:

- Changed default allocator to mimalloc.

crates/zed/Cargo.toml

@@ -66,6 +66,7 @@ libc = "0.2"
 log.workspace = true
 lsp = { path = "../lsp" }
 menu = { path = "../menu" }
+mimalloc = "0.1"
 node_runtime = { path = "../node_runtime" }
 notifications = { path = "../notifications" }
 num_cpus = "1.13.0"