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.

Cargo.lock

@@ -4093,6 +4093,16 @@ version = "0.2.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f7012b1bbb0719e1097c47611d3898568c546d597c2e74d66f6087edd5233ff4"
 
+[[package]]
+name = "libmimalloc-sys"
+version = "0.1.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3979b5c37ece694f1f5e51e7ecc871fdb0f517ed04ee45f88d15d6d553cb9664"
+dependencies = [
+ "cc",
+ "libc",
+]
+
 [[package]]
 name = "libsqlite3-sys"
 version = "0.26.0"
@@ -4422,6 +4432,15 @@ dependencies = [
  "objc",
 ]
 
+[[package]]
+name = "mimalloc"
+version = "0.1.39"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fa01922b5ea280a911e323e4d2fd24b7fe5cc4042e0d2cda3c40775cdc4bdc9c"
+dependencies = [
+ "libmimalloc-sys",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.17"
@@ -10285,6 +10304,7 @@ dependencies = [
  "log",
  "lsp",
  "menu",
+ "mimalloc",
  "node_runtime",
  "notifications",
  "num_cpus",