meson: Build pgevent as shared_module rather than shared_library

meson: Build pgevent as shared_module rather than shared_library

This matches the behavior of the makefiles and the old MSVC build
system.  The main effect is that the build result gets installed into
pkglibdir rather than bindir.  The documentation says to locate the
library in pkglibdir, so this makes the code match the documentation
again.

Reviewed-by: Ryohei Takahashi (Fujitsu) <r.takahashi_2@fujitsu.com>
Discussion: https://www.postgresql.org/message-id/flat/TY3PR01MB118912125614599641CA881B782522%40TY3PR01MB11891.jpnprd01.prod.outlook.com