Option to choose formatter, Black as default

[akaihola]

There are feature requests for

• using Ruff or Blue instead of Black as the formatter
  • Support ruff format as a formatter #521
  • Support for blue as a formatter #251
• just running isort and/or flynt but no formatter at all
  • [DESIGN] support different code formatters, allow to skip formatting #304

For these, a command line and configuration option for choosing (or omitting) the formatter is needed.

In discussion for #304, a --format= or --formatter= option was proposed. The --format= variant was preferred since it corresponds to the name of the --lint= option (not --linter=). On the other hand, now that linting is moving to the separate Graylint package anyway, maybe we could also use the more descriptive variant --formatter=.

In the first phase, we would just add --formatter= with the default value of --formatter=black and no other supported options. This would pave the way for adding other formatters.

In pyproject.toml, a corresponding formatter = option would be added, with formatter = "black" as the default.

See also:

• many-formatters sub-project