Problem: Currently Neovim only searches for lpeg.dylib, liblpeg.dylib,
etc. on MacOS, but a normal installation of lpeg will produce lpeg.so
instead. There are explicit workarounds for this by modifying lpeg's
package in Homebrew, and will shortly be another in spack.
Solution: also search for lpeg.so even if that's not the platform
default, because Lua and luarocks use it anyway.