Add the FORGEJO__ prefix as equivalent to GITEA__ when interpreted by
environment-to-ini. It is used when running the Forgejo container like
so:
docker run --name forgejo -e FORGEJO__security__INSTALL_LOCK=true \
-d codeberg.org/forgejo/forgejo:1.19
(cherry picked from commit 6cd61e2ab7)
(cherry picked from commit 62cae8cc6a)
(cherry picked from commit aee1afc509)
(cherry picked from commit 6ba563cd9b)
(cherry picked from commit 6429b20f4a)
(cherry picked from commit dd545aa077)
(cherry picked from commit 63a00e573e)
(cherry picked from commit 8e35a50b91)
(cherry picked from commit 26e8fb6cd9)
(cherry picked from commit 56bbf644be)
(cherry picked from commit 4d0a8c8640)
(cherry picked from commit b58f775fa2)
(cherry picked from commit f4b6fa7a93)
(cherry picked from commit 4eca363082)
Replace #19857Close#19856Close#10311Close#10123
Major changes:
1. Move a lot of code from `environment-to-ini.go` to `config_env.go` to
make them testable.
2. Add `__FILE` support
3. Update documents
4. Add tests
