Phase Extension Convention

Once we start writing more custom phase extensions to the repository, we need to set up a convention to follow. Rules like
- The name of phase `.bzl` file (e.g. `phase_scalafmt.bzl`)
- The name of extension `.bzl` file (e.g. `phase_scalafmt_ext.bzl`)
- The object names in extension file
- Extension object (e.g. `ext_scalafmt`)
- Rule to provide `ScalaRulePhase` (e.g. `scalafmt_singleton`)

Once we have agreements, I am happy to write it to a README.

Start by reviewing the phase and extension naming examples in the issue, including phase_scalafmt.bzl, phase_scalafmt_ext.bzl, ext_scalafmt, and scalafmt_singleton. Agree on the convention for these names, then record the decision in a README; done means the README documents the agreed rules clearly.

Written by the indexing model from the issue text.