[DESIGN] support different code formatters, allow to skip formatting

See also: many-formatters sub-project

Currently there's no way to run Darker and have it just do, say, isort and flynt but not reformat code using Black.

Also, there are open requests for replacing Black with another formatter or expanding the choices of formatters/linters:

We could expand Darker's behavior in stages:

extend configuration parameters:
- add --black/--no-black; see
  - Add --black / --no-black options #449
  - Make boolean configuration options overridable #301
- OR: add --format=black/--format=blue/--format=ruff/--format=none
  - see Option to choose formatter, Black as default #562
add support for blue, Ruff and other formatters; see
- Add Blue as an alternative reformatter #309
- Support ruff format as a formatter #521
add another console entry point which defaults to --format=none (what would be a good name for that?)
change default for darker to --format=none in version 2.0.0
- see Run no formatter by default #450

For the option name, I've considered --format, --formatter, --reformat and --reformatter:

Black calls itself a "formatter", so I discarded --reformat and --reformatter (also for needless verbosity)
Linters are chosen with --lint, not --linter, so for consistency, I'd prefer --format over --formatter

At some point after making --format=none the default, black should be removed as a hard dependency. I can see two alternatives:

For the --black/--no-black/--blue/--no-blue design choice, some pros and cons are:

🟢👍 --black: can enable multiple formatters (although what about the running order?)
🟢👍 --black: consistent with --isort and --flynt
🟢👍 --black: shorter to type

and for --format={black|blue|none}:

🟢👍 --format=: consistent with --lint=
🔴👎 --format= inconsistent with --lint= since --format=black or --format=blue don't really specify a command line
🔴👎 --format=: more verbose

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