refactor file handling in WASM wrapper

[hanayik]

This PR refactors the file handling in the JavaScript wrapper to allow similar behaviour when files come from <input> html elements using webkitdirectory and when files come from users drag and drop events where the files will be from event.dataTransfer.items.

Unfortunately, the latter case requires some custom event handling by the downstream developer to recursively read files from directories.

We have this same constraint in Niivue, but we attach event handlers directly to the canvas element there. In the case of dcm2niix, it does not care about html elements or their events, so that handling has to be taken care of prior to passing files in.

The changes in this PR make that process slightly less painful though.

two new methods on the Dcm2niix class are available:

• inputFromWebkitDirectory
• inputFromDropItems
• also: file from event.dataTransfer.items (i.e. dropped files) must have the custom property _webkitRelativePath appended.

This all works, and is demonstrated in the live demo here: https://niivue.github.io/niivue-dcm2niix/

and in the demo web app repo here: https://github.com/niivue/niivue-dcm2niix