Import source for g-ir-generate and g-ir-inspect from gobject-introspection.git

Like !3830 (merged), this was done using:

git clone git@ssh.gitlab.gnome.org:GNOME/gobject-introspection.git
cd gobject-introspection
git filter-repo --path 'tools/generate.c' --path 'tools/g-ir-inspect.c' --path 'src/generate.c'
git filter-repo --to-subdirectory-filter girepository --force

then in my glib.git checkout:

git remote add local ~/gobject-introspection/
git remote update local
git checkout local/main
git checkout -b 3155-import-generate-and-inspect
git rebase -i --autosquash main
git push --force-with-lease --atomic pwithnall HEAD

Following MRs will rework this code; the point of this MR is to import the history.

Helps: #3155 (closed)