feat(argtype): :alone - the value of an option written without one

An optional argument has three values: the default (the name is absent),
the argument type's :alone value (the name is written alone), and a
written value.

:alone is declared by the argument type only, never by a klammer's
parameter declaration -- a default is what one klammer means by silence,
but a bare option name must read the same way in every klammer.  The bool
type declares :alone true, which is the whole of the convention that a
bare boolean option means true; there is no boolean special case in the
engine.  A type whose pattern matches running text cannot declare :alone,
since an option's value runs to the next bar or option name and would
swallow the following text.

kdesc and `ktext -m` now show [default: X] and [alone: Y] per argument
type.

In the Standard Klammer Set: @code :number becomes a bool (it was an
untyped string tested only for truthiness, so a bare :number was a
no-op); decimal_mark declares :alone comma; table_hline and table_vline
declare :alone all.  The 35 bools that default false gained the bare form
for free.

Tests: tst/alone_test.sh (21 cases) joins the shipped suite.

(from dev 6024f49c2859)

tst/Makefile

@@ -1,10 +1,12 @@
 # Klammertext distribution test suite (subset).
 #
-# Runs the six shell regression suites:
+# Runs the seven shell regression suites:
 #   cond_test.sh     — @cond argument delimitation
 #   deftype_test.sh  — the four klammer definition modes + redefinition table
 #   escape_test.sh   — target character escaping and quoted specials
 #   filename_test.sh — filenames with spaces (quoting, " / " lists, rescue)
+#   alone_test.sh    — an optional argument's three values (default, the
+#                      argument type's :alone value, a written value)
 #   modulepath_test.sh — @eval finds modules beside the file that names them
 #   editor_test.sh   — editor support (doc/edit): indentation and table
 #                      alignment; needs python3, uses Emacs when installed
@@ -17,5 +19,6 @@ test:
 	./deftype_test.sh
 	./escape_test.sh
 	./filename_test.sh
+	./alone_test.sh
 	./modulepath_test.sh
 	./editor_test.sh