The loader package's `ensure_wasi_sdk_exists` private method checks for
the wasi-sdk, fetching it if it can't be found. This logic was
re-implemented in xtask for `build-wasm-stdlib`, but without the
fetching functionality. We can have nice things in xtask too! Rather
than make this function a public member of `tree-sitter-loader`, we
just re-implement and leave a nice comment asking people to keep the
two in sync.
