Add support for HTMLInputElement:webkitdirectory API (principally for Firefox)

[Jaifroid]

See kiwix/kiwix-js#1032 for the upstream version of this. It is much easier to add this here, since we already support the File System Access API in KJSWL, and the webkitdirectory API is a "poormans" version of that API. It is mostly a case of inserting a few hooks, and dealing with loading split archives from any picked directory.

Once the directory is picked, the API is indistinguishable from the File System Access API. The main difference is having to pick the directory of ZIM archives at the start of each session.