wasi-threads: build the crate in the CLI application by default (#5782)

This change adds the `wasmtime-wasi-threads` crate as a default crate
for the CLI application. This is no change for embedders of Wasmtime:
they would still have to include `wasmtime-wasi-threads` manually.
Enabling the crate by default in the CLI application has several
benefits, e.g., that it is simpler to experiment with and that it will
be part of more test runs (and thus bugs can be discovered more
quickly). Users will still have to add
`--wasi-modules=experimental-wasi-threads` to enable wasi-threads on the
command line.

Cargo.toml

@@ -201,6 +201,7 @@ default = [
   "wasmtime/parallel-compilation",
   "vtune",
   "wasi-nn",
+  "wasi-threads",
   "pooling-allocator",
 ]
 jitdump = ["wasmtime/jitdump"]