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)

mac/argtype.cpp

@@ -7,13 +7,15 @@
 #include "util.h"
 
 Argtype::Argtype(std::string name, std::string desc, std::string symbolic_pattern, std::string pattern,
-                 std::string default_value, std::string python_cast, modify_string_f python_format,
+                 std::string default_value, std::string alone_value,
+                 std::string python_cast, modify_string_f python_format,
                  const Locator& loc)
     : m_name(name)
     , m_desc(desc)
     , m_symbolic_pattern(symbolic_pattern)
     , m_pattern(pattern)
     , m_default(default_value)
+    , m_alone(alone_value)
     , m_python_cast(python_cast)
     , m_python_format(python_format)
     , m_regex(std::regex(pattern))