girepository: Add g-ir-compiler source code

Preparation for !3797 (merged), this imports the git history of compiler.c from gobject-introspection using git subtree split and git filter-repo:

In gobject-introspection (fresh clone)...

git-filter-repo --path 'tools/compiler.c' --path 'src/compiler.c'
git filter-repo --to-subdirectory-filter girepository --force
git remote add ssh git@ssh.gitlab.gnome.org:GNOME/gobject-introspection.git
git push -fu ssh main:ewlsh/tools-only

In glib (with a remote gi pointing to gobject-introspection)...

git fetch gi
git checkout gi/ewlsh/tools-only
git checkout -b ewlsh/gir-compiler-source
git rebase ssh/main
git push -fu ssh ewlsh/gir-compiler-source