JS/base: provide implementation for mkdir (#22374)

Closed Josh Meredith requested to merge wip/js-mkdir into master

See ticket #22374

This MR implements mkdir for JavaScript targets (linked as h$mkdir because of how the JavaScript backend treats c imports). The implementation lives in base/jsbits/base.js.

The tests openFile003 and TargetContents are partially fixed by this MR, but are still erroring due to #22362 (closed)

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