Adds mimalloc as default feature. (#10378)

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This PR makes `mimalloc` the default allocator, it has been the default
on the nightly builds of nushell now for a couple of months and the
performance improvements are quite nice, measuring upwards of 30% faster
startup time on Windows, and a bit smaller on Linux,
https://github.com/nushell/nushell/pull/9415

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Cargo.toml

@@ -112,7 +112,7 @@ plugin = [
 	"nu-protocol/plugin",
 	"nu-engine/plugin",
 ]
-default = ["plugin", "which-support", "trash-support", "sqlite"]
+default = ["plugin", "which-support", "trash-support", "sqlite", "mimalloc"]
 stable = ["default"]
 wasi = ["nu-cmd-lang/wasi"]
 # NOTE: individual features are also passed to `nu-cmd-lang` that uses them to generate the feature matrix in the `version` command