[3.13] GH-121521: Detect when wasmtime is not installed in `Tools/wasm/wasi.py` (GH-121522) (GH-121558)

GH-121521: Detect when wasmtime is not installed in `Tools/wasm/wasi.py` (GH-121522)
(cherry picked from commit f62161837e68c1c77961435f1b954412dd5c2b65)

Co-authored-by: Brett Cannon <brett@python.org>