Bug 35642: Upgrade Font Face Observer library from v2.0.3 to v2.3.0

Bug 35642: Upgrade Font Face Observer library from v2.0.3 to v2.3.0

This patch upgrades the Font Face Observer library from v2.0.3 to the
latest release, v2.3.0. The file is moved to its own directory for
consistency's sake.

To test, apply the patch and clear your browser cache.

- Load any page in the OPAC.
- The custom font we supply for the OPAC (Noto Sans) should load
  correctly.
- If you want to see more about the functionality of the library, use
  your browser's developer tools to simulate a slow connection.
  - The page should load with a default sans-serif font first, and then
    the downloaded font will be applied.
  Note: If you throttle your connection speed *too* much, the loading of
  the custom font will time out and you'll only see the default
  sans-serif font.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>