This will make it easier for everyone to update subprojects and the like by running `meson subprojects update` from the source directory. We'll need to add this to the documentation I'm just not sure exactly where right now.
Testing Done:
Ran `meson subprojects update` from the source directory with `subprojects/gplugin` on the default branch and the v0.31.0 tag. Verified that meson will update to whatever revision is specified and if the revision doesn't exist it will pull from the remote.