feat: Add flame create, the ignite_cli scaffolding as a flutter create style command

[spydon]

Description

Stacked on #4057. This brings the scaffolding of ignite_cli
into flame_cli as flame create, structured like flutter create: no interactive prompts, a
positional output directory, sensible defaults and options to change them.

flame create my_game
cd my_game
flame run

How it works:

• flutter create --empty generates the platform folders and the pubspec.yaml, with
  --project-name, --org, --description, --platforms and --overwrite passed through.
• The files of the chosen template (--template simple|basics|example, basics by default) are
  written on top, plus an analysis_options.yaml that includes flame_lint.
• flutter pub remove flutter_lints and flutter pub add flame dev:flame_lint [dev:flame_test] [--packages ...] set up the dependencies. Using pub add means the game always starts on the
  newest compatible versions, without a version list baked into the CLI or fetched from pub.dev
  the way ignite did. --flame-version ^1.30.0 pins a constraint when needed.
• The project name defaults to the directory name and is validated with the same rules as
  flutter create (a lowercase Dart identifier that is not a keyword or a clashing package name),
  with a suggestion when it is invalid. An existing project is not touched without --overwrite.

The three templates are updated versions of ignite's:

• simple: an empty FlameGame in a GameWidget.
• basics: a world with a component that starts moving when tapped, and a FlameTester widget
  test for it.
• example: a small complete game over a few files: a world, a camera with a score in the
  viewport, a player steered with the arrow keys or by tapping, stars to collect through
  collision detection, and four testWithGame tests.

The command shows up in a new "Creating a game" group at the top of flame --help.

Tests use a fake flutter process and check the exact command sequence, the written files, the
options, the overwrite guard, the validation and the failure path. I also created a game from each
template with the real flutter: all three pass flutter analyze, basics and example pass
flutter test, and the example runs under flame run on flutter-tester.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

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