cmds.projects.CmdCreateFile: Add --field-separator option

List values in a rendered template are joined with a fixed separator of
",\n" (comma plus newline), and there is no way to change that. Templates
sometimes want a different separator, e.g. a plain newline or a single-line
comma-separated list.

Add a --field-separator option that controls how list values are joined
in the rendered output. %n expands to a newline, and the default remains
",\n" for both formats. render_tmpl() applies the separator to the rendered
template, while render_pyright() rejects any other value: the built-in
pyrightconfig.json template is a fixed JSON document, and no alternative
separator is supported for it.

Update the help output test expectation accordingly.

Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.85.1
Signed-off-by: Jan Lindemann <jan@janware.com>

test/unit/python/jw/pkg/cmds/projects/CmdCreateFile/Makefile

@@ -0,0 +1,8 @@
+TOPDIR = ../../../../../../../..
+
+include $(TOPDIR)/make/proj.mk
+include $(JWBDIR)/make/py-run.mk
+
+all:
+
+test: run

test/unit/python/jw/pkg/cmds/projects/CmdCreateFile/test.py

@@ -0,0 +1,25 @@
+from jw.pkg.cmds.projects.CmdCreateFile import CmdCreateFile
+
+# __format_separator() is a private method that does not use self, so an
+# uninitialised instance is a sufficient receiver for the bound method
+inst = CmdCreateFile.__new__(CmdCreateFile)
+fmt = getattr(inst, '_CmdCreateFile__format_separator')
+
+# -- __format_separator --
+
+# A None separator falls back to the format's default
+assert fmt(None, ',\n') == ',\n'
+
+# %n expands to a newline, wherever it appears in the separator
+assert fmt('%n', ',\n') == '\n'
+assert fmt(',%n', '\n') == ',\n'
+assert fmt('a%n b', '') == 'a\n b'
+
+# Several %n expand independently
+assert fmt('%n%n', '') == '\n\n'
+
+# Anything that is not %n passes through untouched
+assert fmt('%', '') == '%'
+assert fmt('%x', '') == '%x'
+
+print('All CmdCreateFile tests passed')