This is a pretty significant release!
This version includes some clever tricks to ensure that pfSense can still boot even if the package is uninstalled with tunnels still assigned as interfaces. This requires leaving a very small script that will attempt to bootstrap these interfaces by temporarily creating them as loopback interfaces of the same names. In this way, pfSense can boot and the WireGuard package can be reinstalled, which at that time the temporary loopbacks are destroyed and replaced with true WireGuard interfaces.
This has also been tested on pfSense 2.5.1-RC and 2.6.0 snapshots.
Get all of these packages onto your pfSense box and run
pkg add pfSense-pkg-WireGuard-0.0.4_0.txz