cmds.projects.CmdCreateFile: Fix --format help

The possible values of --format are defined by an Enum which is never
really used as such. Derive them from class introspection instead, i.e.
offer all formats that have a corresponding render_<format>() name.

Also, turn the option into real argparse-backed choices, and make the
argument mandatory, because that reflects the reality of the implementation
- there is no default.

Signed-off-by: Jan Lindemann <jan@janware.com>

test/integration/jw-pkg/help/test-expected.txt

@@ -332,7 +332,7 @@ List available commands
 options:
   -h, --help  show this help message and exit
 ============= Running: jw-pkg.py -t ../../../.. --log-level info projects create-file --help
-usage: jw-pkg.py projects create-file [-h] [--format FORMAT]
+usage: jw-pkg.py projects create-file [-h] --format {pyright,tmpl}
                                       [--search-path SEARCH_PATH]
                                       [--template-name TEMPLATE_NAME]
                                       [--quote] [-f KEY=VALUE]
@@ -345,7 +345,8 @@ positional arguments:
 
 options:
   -h, --help            show this help message and exit
-  --format FORMAT       pyright (default: None)
+  --format {pyright,tmpl}
+                        Output format, one of: pyright, tmpl
   --search-path SEARCH_PATH
                         Template search path, colon separated (default:
                         /etc/opt/jw-pkg/templates)