Initialize the libzim worker if specific criteria are met

[Jaifroid]

This is a sub-issue of #769. The currently proposed criteria for initializing the libzim worker are:

• We are in ServiceWorker mode
• We are using WASM (not ASM)
• The ZIM archive has a Xapian full-text search index (Add a Directory Listing object to the Zim Archive metadata for Xapian indices #928)
• The archive is not split (this could be reviewed once split ZIM reading is working with the JS libzim port)

[Jaifroid]

This issue should also update the API panel to show that libzim has been successfully initialized.

[Jaifroid]

In practice, support for WASM implies support for Service Worker. But it should not be necessary to turn off full text search if the user happens to switch to jQuery mode.