Add a Meson devenv configuration.
This allows running meson devenv as defined at https://mesonbuild.com/Commands.html#devenv

Ran meson devenv -C build pidgin3 and meson devenv -C build finch3 on a system without either installed (even from this build).
Note stock icons don't seem to work with XDG_DATA_DIRS, which is why the other review requests were needed. If anything still has stock icons, they'll be broken without an install.