Use cross-OS way to delete build output

[jeroenheijmans]

Fixes #175

Saw #175 come by in my feed. Have successfully solved this with the rimraf npm package which is one of the suggestions in the top Stack Overflow post on the subject, but I personally agree with the lower voted answer that for such a simple task using a package with less dependencies seems preferable.

I've tested this on my Windows 10 machine, not yet on any Linux VM (nor did I see any CI or GitHub actions in this repo to verify it?). Will first see what the maintainers think of this approach.

If you don't like this approach: no harm done! Your call.

(I realize the title of #175 is "Powershell support", but I took it to be in general about cross-OS and cross-shell support. It should also work with cmd, right?)

[kaelig]

👋 This approach is creative, but comes at the expense of obscuring the intent behind the code.

My 2c: shx and rimraf would be more idiomatic.