CMake build: find enchant 2.x binary and lib

CMake build: find enchant 2.x binary and lib

For example, see the Fedora file lists for the package
enchant2-devel [1] as well as the Debian Sid file lists for the
packages libenchant-2-dev [2] and enchant-2 [3]. The packages for
Ubuntu 20.04 are similar. Our autotools build has had support for
finding enchant 2.x since 71977769.

Searching for enchant-2 first is consistent with what we do for
autotools (see 1ae537c5).

This commit is related to #10986.

[1] https://www.lyx.org/trac/ticket/10986#comment:28
[2] https://packages.debian.org/sid/amd64/libenchant-2-dev/filelist
[3] https://packages.debian.org/sid/amd64/enchant-2/filelist