Recursively create output directory

[wleslie]

Creates parent dirs of output directory if necessary. This makes the behavior consistent with the
rest of Webpack (for example, the output.path config setting creates parent dirs as needed).

[wleslie]

Note that wasm-pack itself creates its output dir recursively, but wasm-pack-plugin also attempts to create this directory first, non-recursively, so it will fail if the parent dir does not exist. This change fixes that.

(Aside: due to #74, wasm-pack and wasm-pack-plugin have different ideas about what the output dir is relative to, so two directories end up getting created.)

[xtuc]

Could you please rebase? but change lgtm

[xtuc]

Published in @wasm-tool/[email protected]