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/argument_set.cpp

@@ -417,6 +417,16 @@ Parameter_set::value_map(
     }
     auto optional_values = check_optional(optional, loc);
     for (auto [key, value] : optional_values) {
+        // check_optional returns only the options that were actually
+        // written, so an empty value here means the name was written alone
+        // (":number" rather than ":number 10") — distinct from the option
+        // being absent, which is filled from the default below.  The
+        // argument type supplies the alone value; bool declares "true",
+        // which is what makes a bare boolean option mean true.
+        const Parameter* parameter = find(key);
+        if (value.empty() && parameter && !parameter->m_argtype.m_alone.empty()) {
+            value = parameter->m_argtype.m_alone;
+        }
         values[key] = value;
     }
     for (auto opt : m_optional) {