Add a Meson devenv configuration.

This allows running `meson devenv` as defined at https://mesonbuild.com/Commands.html#devenv

Testing Done:
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.

Reviewed at https://reviews.imfreedom.org/r/973/