The INI package has many bugs and quirks, and in fact it is
unmaintained.
This PR is the first step for the INI package refactoring:
* Use Gitea's "config_provider" to provide INI access
* Deprecate the INI package by golangci.yml rule
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