Buildifier - Create a more sensible "flags" system and experimental features

Many PRs in this repository fall into specific categories, e.g. implementation of warnings / formatting changes which are not possible to enable by default, or just resolving very specific needs.

Right now there's no branching or flags-system for implementing these besides submitting them as part of core buildifier / buildozer. If we defined a separate directory and interface for experimental warnings we could simplify the process of getting custom checks merged, and through this enable better support for buildtools users.

The issue does not identify files, tests, or entry points. Start by mapping how Buildifier and Buildozer currently register and enable warnings, then review existing warning-related changes and interfaces. Done means an agreed separate directory and interface exists for experimental warnings, with a clear way to enable them without changing core defaults.

Written by the indexing model from the issue text.